Online Etymology Dictionary

ibex (n.): "chamois, wild goat of the Alps and Apennines," c. 1600, from Latin ibex, which probably is from a pre-Latin Alpine language. The German Steinbock.
ibid. (adv.): "at the place or in the book already mentioned" (used to avoid repetition of references), 1660s, abbreviation of Latin ibidem "in the same place, just there," from ibi "there," pronominal adverb of place, + demonstrative suffix -dem. Also ibid, but properly with the period.
ibis (n.): stork-like bird, late 14c., from Latin ibis (plural ibes), from Greek ibis, from Egyptian hab, a sacred bird of Egypt.
ibogaine (n.): nerve stimulant, 1901, from French ibogaine, from iboga, Congolese name of the shrub from which the chemical is extracted, + chemical suffix -ine (2).
ICBM (n.): also I.C.B.M., 1955, initialism (acronym) for Inter-Continental Ballistic Missile. IBM in the same sense is from 1954.
iceberg (n.): 1774, "glacier humped like a hill;" 1820 as "detached piece of a glacier or ice pack at sea," partial loan-translation of Dutch ijsberg, literally "ice mountain," from ijs "ice" (see ice (n.)) + berg "mountain" (see barrow (n.2)). Similar formation in Norwegian isberg, Danish isbjerg.

Earlier English terms were sea-hill (1690s), island of ice (1610s). Phrase tip of the iceberg in a figurative sense (in allusion to most of it being unseen underwater) first recorded 1962. Iceberg lettuce attested from 1893, apparently originally a trade name.
ichneumon (n.): 1570s, "weasel-like animal of Egypt," from Latin ichneumon, from Greek ikhneumon "ichneumon," literally "searcher, tracker," perhaps so called because it hunts crocodile eggs, from ikhneuein "hunt for, track," from ikhnos "a track, footstep, trace, clue," of unknown origin. Used by Aristotle for a species of wasp that hunts spiders (a sense attested in English from 1650s).
ichor (n.): "ethereal fluid that serves for blood in the veins of the gods," 1630s, from French ichor (16c.) or Modern Latin ichor, from Greek ikhor, of unknown origin, possibly from a non-Indo-European language. The fluid that serves for blood in the veins of the gods. Related: Ichorous.
ichthyosaur (n.): extinct aquatic reptile, 1830, Modern Latin, from Latinized form of Greek ikhthys "fish" (see ichthyo-) + -saurus. Related: Ichthyosaurus (1819); ichthyosaurian.
icicle (n.): early 14c., isykle, from is "ice" (see ice (n.)) + Middle English ikel, a word that by itself meant "icicle," from Old English gicel "icicle, ice" (found in compounds, such as cylegicel "chill ice"), from Proto-Germanic *jekilaz (source also of Old Norse jaki "piece of ice," diminutive jökull "icicle, ice; glacier;" Old High German ihilla "icicle"), from PIE *yeg- "ice" (source also of Middle Irish aig "ice," Welsh ia). Dialectal ickle "icicle" survived into 20c.
Icknield Way: prehistoric trackway from Norfolk to Dorset, Old English Iccenhilde, Icenhylte (903), which is of unknown meaning and origin. There was a Romano-British Iceni tribe in modern Norfolk. The name was transferred 12c. to the ancient Roman road from Burton on the Water to Templeborough.
icky (adj.): 1935, American English, probably from icky-boo (c. 1920) "sickly, nauseated," which probably is a baby talk elaboration of sick (adj.). Originally a swing lover's term for more sentimental jazz music; in general use, "sticky and repulsive," from 1938. Also a noun, "person with conventional taste in jazz," 1937.
icon (n.): also ikon, 1570s, "image, figure, picture," also "statue," from Late Latin icon, from Greek eikon "likeness, image, portrait; image in a mirror; a semblance, phantom image;" in philosophy, "an image in the mind," related to eikenai "be like, look like," from PIE *weik- (3) "to be like." The specific Eastern Church sense is attested from 1833 in English. Computing sense first recorded 1982.
iconoclast (n.): "breaker or destroyer of images," 1590s, from French iconoclaste and directly from Medieval Latin iconoclastes, from Late Greek eikonoklastes, from eikon (genitive eikonos) "image" + klastes "breaker," from klas- past tense stem of klan "to break" (see clastic).

Originally in reference to those in the Eastern Church in 8c. and 9c. whose mobs of followers destroyed icons and other religious objects on the grounds that they were idols. Applied to 16c.-17c. Protestants in Netherlands who vandalized former Catholic churches on similar grounds. Extended sense of "one who attacks orthodox beliefs or cherished institutions" is first attested 1842.
idea (n.): late 14c., "archetype, concept of a thing in the mind of God," from Latin idea "Platonic idea, archetype," a word in philosophy, the word (Cicero writes it in Greek) and the idea taken from Greek idea "form; the look of a thing; a kind, sort, nature; mode, fashion," in logic, "a class, kind, sort, species," from idein "to see," from PIE *wid-es-ya-, suffixed form of root *weid- "to see" (see vision). In Platonic philosophy, "an archetype, or pure immaterial pattern, of which the individual objects in any one natural class are but the imperfect copies, and by participation in which they have their being" [Century Dictionary].

Meaning "mental image or picture" is from 1610s (the Greek word for it was ennoia, originally "act of thinking"), as is the sense "concept of something to be done; concept of what ought to be, differing from what is observed." Sense of "result of thinking" first recorded 1640s. Idée fixe (1836) is from French, literally "fixed idea." Through Latin the word passed into Dutch, German, Danish as idee, which also is found in English dialects. The philosophical sense has been somewhat further elaborated since 17c. by Descartes, Locke, Berkeley, Hume, Kant. Colloquial big idea (as in what's the ...) is from 1908.
ideal (adj.): early 15c., "pertaining to an archetype or model," from Late Latin idealis "existing in idea," from Latin idea in the Platonic sense (see idea). Senses "conceived as perfect; existing only in idea," are from 1610s. Ideagenous "generating ideas" (1839) is a word from early psychology, apparently coined by Dr. Thomas Laycock, house surgeon to York County Hospital ["Edinburgh Medical and Surgical Journal," vol. 52].
identical (adj.): 1610s as a term in logic; general sense of "being the same or very similar" is from 1630s, from Medieval Latin identicus "the same," from Late Latin identitas "identity, sameness," ultimately from comb. form of Latin idem "the same" (see idem). Replaced Middle English idemptical (late 15c.), from Medieval Latin idemptitas "identity," from Latin idem. Related: Identically.
identify (v.): 1640s, "regard as the same," from French identifier, from identité (see identity). Sense of "determine the identity of, recognize as or prove to be the same" first recorded 1769. Meaning "make one (with), associate (oneself), regard oneself as being the essence of" is from 1780. Sense of "serve as means of identification" is attested by 1886. Related: Identified; identifying.
identity (n.): c. 1600, "sameness, oneness, state of being the same," from Middle French identité (14c.), from Medieval Latin identitatem (nominative identitas) "sameness," ultimately from Latin idem (neuter) "the same" (see idem). [For discussion of Latin formation, see entry in OED.] Earlier form of the word in English was idemptitie (1560s), from Medieval Latin idemptitas. Term identity crisis first recorded 1954. Identity theft attested from 1995.
ides (n.): "middle day of a Roman month," early 14c., from Old French ides (12c.), from Latin idus (plural) "the ides," a word perhaps of Etruscan origin. In the Roman calendar the eighth day after the nones, corresponding to the 15th of March, May, July, and October; the 13th of other months. "Debts and interest were often payable on the ides" [Lewis].
idiom (n.): 1580s, "form of speech peculiar to a people or place;" meaning "phrase or expression peculiar to a language" is from 1620s; from Middle French idiome (16c.) and directly from Late Latin idioma "a peculiarity in language," from Greek idioma "peculiarity, peculiar phraseology" (Fowler writes that "A manifestation of the peculiar" is "the closest possible translation of the Greek word"), from idioumai "to appropriate to oneself," from idios "personal, private," properly "particular to oneself."

This is from PIE *swed-yo-, suffixed form of root *s(w)e-, pronoun of the third person and reflexive (referring back to the subject of a sentence), also used in forms denoting the speaker's social group, "(we our-)selves" (source also of Sanskrit svah, Avestan hva-, Old Persian huva "one's own," khva-data "lord," literally "created from oneself;" Greek hos "he, she, it;" Latin suescere "to accustom, get accustomed," sodalis "companion;" Old Church Slavonic svoji "his, her, its," svojaku "relative, kinsman;" Gothic swes "one's own;" Old Norse sik "oneself;" German Sein; Old Irish fein "self, himself").
[G]rammar & idiom are independent categories; being applicable to the same material, they sometimes agree & sometimes disagree about particular specimens of it; the most can be said is that what is idiomatic is far more often grammatical than ungrammatical, but that is worth saying, because grammar & idiom are sometimes treated as incompatibles .... [Fowler]
idiosyncrasy (n.): c. 1600, from French idiosyncrasie, from Latinized form of Greek idiosynkrasia "a peculiar temperament," from idios "one's own" (see idiom) + synkrasis "temperament, mixture of personal characteristics," from syn "together" (see syn-) + krasis "mixture," from PIE root *kere- "to mix, confuse; cook" (see rare (adj.2)).

Originally in English a medical term meaning "physical constitution of an individual;" mental sense "peculiar mixture" of the elements in one person that makes up his character and personality first attested 1660s. In modern use, loosely, one's whims, habits, fads, or tastes. Sometimes confused in spelling with words in -cracy, but it is from krasis not kratos.
idiot (n.): early 14c., "person so mentally deficient as to be incapable of ordinary reasoning;" also in Middle English "simple man, uneducated person, layman" (late 14c.), from Old French idiote "uneducated or ignorant person" (12c.), from Latin idiota "ordinary person, layman; outsider," in Late Latin "uneducated or ignorant person," from Greek idiotes "layman, person lacking professional skill" (opposed to writer, soldier, skilled workman), literally "private person" (as opposed to one taking part in public affairs), used patronizingly for "ignorant person," from idios "one's own" (see idiom).

In plural, the Greek word could mean "one's own countrymen." In old English law, one who has been without reasoning or understanding from birth, as distinguished from a lunatic, who became that way. Idiot box "television set" is from 1959; idiot light "dashboard warning signal" is attested from 1961. Idiot savant attested by 1870.
idle (adj.): Old English idel "empty, void; vain; worthless, useless," common West Germanic (cognates: Old Saxon idal, Old Frisian idel "empty, worthless," Old Dutch idil, Old High German ital, German eitel "vain, useless, mere, pure"), of unknown origin.

Subsequent developments are peculiar to English: sense "not employed, not doing work" was in late Old English in reference to persons; from 1520s of things; from 1805 of machinery. Meaning "lazy, slothful" is from c. 1300. In Elizabethan English it also could mean "foolish, delirious, wandering in the mind." Idle threats preserves original sense.
idol (n.): mid-13c., "image of a deity as an object of (pagan) worship," from Old French idole "idol, graven image, pagan god" (11c.), from Latin idolum "image (mental or physical), form," especially "apparition, ghost," but used in Church Latin for "false god, image of a pagan deity as an object of worship." This is from Greek eidolon "mental image, apparition, phantom," also "material image, statue," in Ecclesiastical Greek," a pagan idol," from eidos "form, shape; likeness, resemblance" (see -oid).

A Greek word for "image," used in Jewish and early Christian writers for "image of a false god," hence also "false god." The Germanic languages tended to form a word for it from the reverse direction, from "god" to "false god," hence "image of a false god" (compare Old English afgod, Danish afgud, Swedish avgud, Old High German abgot, compounds with af-/ab- "away, away from" (source of off) + god). The older Greek senses sometimes have been used in English. Figurative sense of "something idolized" is first recorded 1560s (in Middle English the figurative sense was "someone who is false or untrustworthy"). Meaning "a person so adored, human object of adoring devotion" is from 1590s.
idolatry (n.): "worship of idols and images," mid-13c., from Old French idolatrie (12c.), from Vulgar Latin idolatria, contraction of Late Latin idololatria (Tertullian), from Ecclesiastical Greek eidololatria "worship of idols," from eidolon "image" (see idol) + latreia "worship, service" (see -latry).
idyll (n.): also idyl, c. 1600, "short, picturesque pastoral poem," from French idylle (16c.) or directly Latin idyllium, from Greek eidyllion "short, descriptive poem, usually of rustic or pastoral type," literally "a little picture," diminutive of eidos "form" (see -oid).
igloo (n.): "dome-shaped Eskimo hut, usually made of blocks of hard snow," 1824, Canadian English, from an Eskimo word for "house, dwelling" (compare Greenlandic igdlo "house"). Of any dome-shaped construction, by 1956.
igneous (adj.): 1660s, "pertaining to or resembling fire," from Latin igneus "of fire, fiery; on fire; burning hot," figuratively "ardent, vehement," from ignis "fire, a fire," extended to "brightness, splendor, glow;" figuratively "rage, fury, passion," from PIE root *egni- "fire" (source also of Sanskrit agnih "fire, sacrificial fire," Old Church Slavonic ogni, Lithuanian ugnis "fire"). Geological meaning "produced by volcanic forces" is from 1791, originally in distinction from aqueous. Earlier in the sense "fiery" were ignean (1630s), ignic (1610s).
ignis fatuus (n.): "will o' the wisp, jack-o-lantern," 1560s, Medieval Latin, literally "foolish fire;" see igneous + fatuous. "It seems to have been formerly a common phenomenon; but is now exceedingly rare" [OED].
ignite (v.): 1660s (trans.), from Latin ignitus, past participle of ignire "set on fire, make red hot," from ignis "fire" (see igneous). Attested earlier as an adjective (1550s). Intransitive sense from 1818. Related: Ignited; igniting.
ignoble (adj.): mid-15c., "of low birth;" 1590s as "not honorable, of low character;" from Middle French ignoble (14c.), from Latin ignobilis "unknown, undistinguished, obscure; of base birth, not noble," from assimilated form of in- "not, opposite of" (see in- (1)) + gnobilis "well-known, famous, renowned, of superior birth" (see noble (adj.)). Related: Ignobly; ignobility.
ignominious (adj.): early 15c., from Middle French *ignominieus or directly from Latin ignominiosus "disgraceful, shameful," from ignominia "disgrace, infamy, loss of a (good) name," from assimilated form of in- "not, opposite of" (see in- (1)) + nomen (genitive nominis) "name" (see name (n.)). The Latin spelling perhaps influenced by words from Old Latin gnoscere "come to know." Related: Ignominiously; ignominiousness.
ignoramus (n.): 1570s, originally an Anglo-French legal term (early 15c.), from Latin ignoramus "we take no notice of, we do not know," first person plural present indicative of ignorare "not to know, take no notice of" (see ignorant). The legal term was one a grand jury could write on a bill when it considered the prosecution's evidence insufficient. Sense of "ignorant person" (1616) came from the title role in George Ruggle's 1615 play in Latin satirizing the ignorance of common lawyers. The plural is ignoramuses as it never was a noun in Latin.
ignorant (adj.): late 14c., "lacking wisdom or knowledge; unaware," from Old French ignorant (14c.), from Latin ignorantem (nominative ignorans) "not knowing, ignorant," present participle of ignorare "not to know, to be unacquainted; mistake, misunderstand; take no notice of, pay no attention to," from assimilated form of in- "not, opposite of" (see in- (1)) + Old Latin gnarus "aware, acquainted with" (source also of Classical Latin noscere "to know," notus "known"), from Proto-Latin suffixed form *gno-ro-, suffixed form of PIE root *gno- "to know" (see know). Also see uncouth.

Form influenced by related Latin ignotus "unknown, strange, unrecognized, unfamiliar." Colloquial sense of "ill-mannered, uncouth, knowing nothing of good manners" attested by 1886. As a noun, "ignorant person," from mid-15c. Related: Ignorantly.
ignore (v.): 1610s, "not to know, to be ignorant of," from French ignorer "be unaware of" (14c.), or directly from Latin ignorare "not to know, be unacquainted; take no notice of, disregard" (see ignorant). The original sense in English is obsolete. Sense of "pass over without notice, pay no attention to" in English first recorded 1801 (Barnhart says "probably a dictionary word"), and OED indicates it was uncommon before c. 1850. Related: Ignored; ignoring.
iguana (n.): large lizard of the American tropics, 1550s, from Spanish, from Arawakan (W.Indies) iguana, iwana, the local name for the lizard.
Foure footed beastes ... named Iuannas, muche lyke vnto Crocodiles, of eyght foote length, of moste pleasaunte taste. [Richard Eden, "Decades of the New World," 1555]
I.H.S.: Old English, from Medieval Latin, representing Greek abbreviation of IHSOUS "Jesus," in which the character -H- is the capital of the Greek vowel eta. The Roman form would be I.E.S. Mistaken for a Latin contraction in the Middle Ages, after its Greek origin was forgotten, and sometimes treated as short for Iesus Hominum Salvator "Jesus Savior of Men." Alternative version I.H.C. (terminal -s- often was indicated in later Greek with a character resembling -c-) is found on vestments from 950 C.E., and may be the source of the H. in slang Jesus H. Christ.
Iliad: from Latin Ilias (genitive Iliadis), from Greek Ilias poiesis "poem of Ilion" (Troy), literally "city of Ilius," the mythical founder. With -ad.
ill (adj.): c. 1200, "morally evil; offensive, objectionable" (other 13c. senses were "malevolent, hurtful, unfortunate, difficult"), from Old Norse illr "evil, bad; hard, difficult; mean, stingy," a word of unknown origin. Not considered to be related to evil. From mid-14c. as "marked by evil intentions; harmful, pernicious." Sense of "sick, unhealthy, diseased, unwell" is first recorded mid-15c., probably from a use similar to that in the Old Norse idiom "it is bad to me." Slang inverted sense of "very good, cool" is 1980s..
illegal (adj.): 1630s, from French illégal (14c.) or directly from Medieval Latin illegalis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin legalis (see legal). Term illegal immigrant first recorded 1892 in American English (illegal immigration is from 1887); used in British English in 1940s in reference to the Jewish movement to Palestine.
illegitimate (adj.): 1530s, "born out of wedlock," formed in English (and replacing earlier illegitime, c. 1500), modeled on Late Latin illegitimus "not legitimate" (see il- + legitimate). Sense of "unauthorized, unwarranted" is from 1640s. Phrase illegitimi non carborundum, usually "translated" as "don't let the bastards grind you down," is fake Latin (by 1965, said to date from c. 1939). Carborundum was a brand of abrasives. Related: Illegitimately.
illicit (adj.): c. 1500, from Old French illicite "unlawful, forbidden" (14c.), from Latin illicitus "not allowed, unlawful, illegal," from assimilated form of in- "not, opposite of" (see in- (1)) + licitus "lawful," past participle of licere "to be allowed" (see licence (n.)). Related: Illicitly.
illiterate (adj.): early 15c., "uneducated, unable to read and write" (originally meaning Latin), from Latin illiteratus "unlearned, unlettered, ignorant; without culture, inelegant," from assimilated form of in- "not, opposite of" (see in- (1)) + literatus "educated," literally "furnished with letters" (see literate). Rendered in Old English as unstæfwis. As a noun meaning "illiterate person" from 1620s. Hence, illiterati (1788, Horace Walpole).
illuminati (n.): 1590s, plural of Latin illuminatus "enlightened" (in figurative sense), past participle of illuminare "light up, make light, illuminate" (see illumination). Originally a name applied to a 16c. Spanish sect (the Alumbrados), then to other sects on the continent; since 1797 used as a translation of German Illuminaten, name of a secret society founded 1776 in Ingolstadt, Bavaria, (repressed there 1785) and holding deistic and republican principles; hence used generally of free-thinkers and sarcastically of those professing intellectual enlightenment (1816). Related: Illuminatism; illuminatist.
illumination (n.): late 14c., "spiritual enlightenment," from Late Latin illuminationem (nominative illuminatio), noun of action from past participle stem of Latin illuminare "to throw into light, make bright, light up;" figuratively, in rhetoric, "to set off, illustrate," from assimilated form of in- "in, into" (see in- (2)) + lumen (genitive luminis) "light," related to lucere "to shine" (see light (n.)). Meaning "action of lighting" in English is from 1560s; sense of "intellectual enlightenment" is from 1630s.
illusion (n.): mid-14c., "mockery, scorning, derision;" late 14c., "act of deception; deceptive appearance, apparition; delusion of the mind," from Old French illusion "a mocking, deceit, deception" (12c.), from Latin illusionem (nominative illusio) "a mocking, jesting, jeering; irony," from past participle stem of illudere "mock at," literally "to play with," from assimilated form of in- "at, upon" (see in- (2)) + ludere "to play" (see ludicrous). Sense of "deceptive appearance" first developed in Church Latin. Related: Illusioned "full of illusions" (1920).
illustration (n.): c. 1400, "a shining;" early 15c., "a manifestation;" mid-15c., "a spiritual illumination," from Old French illustration "apparition, appearance" (13c.) and directly from Latin illustrationem (nominative illustratio) "vivid representation" (in writing), literally "an enlightening," from past participle stem of illustrare "light up, make light, illuminate;" figuratively "make clear, disclose, explain; adorn, render distinguished," from assimilated form of in- "in" (see in- (2)) + lustrare "make bright, illuminate," related to lucere "shine," lux "light" (see light (n.)). Mental sense of "act of making clear in the mind" is from 1580s. Meaning "an illustrative picture" is from 1816.
illustrious (adj.): 1560s, "distinguished by greatness, renowned," from Latin illustris "lighted, bright, brilliant;" figuratively "distinguished, famous," probably a back-formation from illustrare "make light, light up, illuminate," figuratively "embellish, distinguish, make famous" (see illustration). Replaced illustre in same sense (mid-15c.), from Middle French illustre.
im-: variant of in- before -b-, -m-, -p- in the sense of "not, opposite of" (immobile, impersonal; see in- (2)) as well as "in, into" (implant, impoverish; see in- (1)). In some English words it alternates with em- (1).