Online Etymology Dictionary

ideological (adj.): 1797, from ideology + -ical or from ideologic (from French idéologique) + -al (2). Related: Ideologically.
ideologue (n.): 1815, in reference to the French Revolutionaries, from French ideologue, from Greek idea (see idea) + -logos (see -logue). Earlier form was ideologist (1798).
ideology (n.): 1796, "science of ideas," originally "philosophy of the mind which derives knowledge from the senses" (as opposed to metaphysics), from French idéologie "study or science of ideas," coined by French philosopher Destutt de Tracy (1754-1836) from idéo- "of ideas," from Greek idea (see idea) + -logie (see -logy). With connective -o- because the elements are Greek. Destutt published his Eléments d'idéologie 1801-1815.
The term ideology did not become widely employed in the nineteenth century, however, and I have not found that Emerson ever used it. It was only after the appearance of Karl Marx's long unpublished The German Ideology and Karl Mannheim's Ideology and Utopia in the period between the world wars of the twentieth century that the term became an omnipresent one. [Lewis P. Simpson, "Mind and the American Civil War," 1989]
Meaning "systematic set of ideas, doctrines through which the world is interpreted" was in use in English by 1907, earliest in socialist and communist writing, with reference to class; from 1918 it came to be used of socialism and communism themselves (along with fascism) and later more broadly still.
Ideology ... is usually taken to mean, a prescriptive doctrine that is not supported by rational argument. [D.D. Raphael, "Problems of Political Philosophy," 1970]
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].
idio-: word-forming element meaning "one's own, personal, distinct," from Greek idios "own, personal, private, one's own" (see idiom).
idiocrasy (n.): "peculiarity" (physical or mental), 1680s, from Latinized form of Greek idiokrasia, from idios "one's own, personal" (see idio-) + krasis "mixing, tempering," from PIE root *kere- "to mix, confuse; cook" (see rare (adj.2)). Related: Idiocratic.
idiocy (n.): "state of being an idiot," 1520s, from idiot on model of prophecy, etc. Early alternatives included idiotacy (1580s), idiotry (1590s).
idiolatry (n.): "self-worship," 1620s, from idio- "self" + -latry "worship." Related: Idiolater; idiolatrous.
idiolect (n.): one's personal way of using a language, 1948, from idio- "one's own, personal" + second element abstracted from dialect. Idioglottic (1888) has a sense "using words invented in one's mind" (from Greek glotta/glossa "tongue").
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]
idiomatic (adj.): 1712, "peculiar to a certain language," from Latin idiomaticus, from Greek idiomatikos "peculiar, characteristic;" from idios "one's own" (see idiom) + matos "thinking, animated" (see automaton). Meaning "marked by use of idioms" is from 1839.
idiopathy (n.): "primary disease," 1690s, Modern Latin, from medical Greek idiopatheia, from idios "one's own" (see idiom) + -patheia, abstract noun formation from pathos "suffering, disease, feeling" (see pathos). Related: idiopathic.
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.
idiosyncratic (n.): 1756, from idiosyncrasy + -ic. Earlier in same sense was idiosyncratical (1640s). Related: Idiosyncratically.
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.
idiotic (adj.): 1713, from idiot + -ic or from Late Latin idioticus "uneducated, ignorant," in classical Latin, "of an ordinary person," from Greek idiotikos "unprofessional, unskilled; not done by rules of art, unprofessional," from idiotes "unskilled person" (see idiot). Idiotical is from 1640s. Related: Idiotically.
idioticon (n.): "a dictionary of a dialect," 1842, via German, from Latinized form of idiotikon, neuter of Greek idiotikos, from idioma (see idiom).
idiotype (n.): "object typical of a class," 1865; see idio- "distinct" + type (n.). Related: Idiotypic.
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.
idle (v.): late 15c., "make vain or worthless" (trans.), from idle (adj.). Meaning "spend or waste (time)" is from 1650s. Meaning "cause to be idle" is from 1788. Intrans. sense of "run slowly and steadily without transmitting power" (as a motor) first recorded 1916. Related: Idled; idling. As a noun, 1630s of persons, 1939 of an engine setting.
idleness (n.): Old English idelnes "frivolity, vanity, emptiness; vain existence;" see idle (adj.) + -ness. Old English expressed the idea we attach to in vain by in idelnisse. In late Old English it began to acquire its sense of "state of being unoccupied, doing no work, or indolent." Similar formation in Old Saxon idilnusse, Old Frisian idlenisse, Old High German italnissa. Spenser, Scott, and others use idlesse to mean "condition of being idle" in a positive sense, as a pleasure.
idler (n.): "one who spends his time in inaction," 1530s, agent noun from idle (v.).
idly (adv.): Old English idellice "vainly;" see idle + -ly (2). From late 14c. as "in an idle or indolent way."
Ido: 1908, artificial language devised in 1907 and based on Esperanto; from Ido -ido "offspring," suffix representing Latin -ida, Greek -ides.
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.
idolater (n.): late 14c., ydolatrer "idol-worshipper," from Old French idolatre, contracted from Late Latin idololatres, from Ecclesiastical Greek eidololatres "idol-worshipper," related to eidolatria (see idolatry).
idolatrous (adj.): 1540s, from idolater + -ous. Related: Idolatrously.
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).
idolization (n.): 1773, noun of action from idolize.
idolize (v.): 1590s, "to admire excessively," from idol + -ize. Literal sense "worship as an idol" is from 1660s. Related: Idolized; idolizing.
Idumaean (adj.): in reference to the biblical Edom, from Latin Idumaea "Edom," from Greek Idoumaia, from Hebrew Edom.
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).
idyllic (adj.): "full of natural, simple charm," 1831, literally "suitable for an idyll" from French idyllique or else from idyll + -ic. In late 18c. it meant "pertaining to an idyll."
if (conj.): Old English gif (initial g- in Old English pronounced with a sound close to Modern English -y-) "if, whether, so," from Proto-Germanic *ja-ba (source also of Old Saxon, Old Norse ef, Old Frisian gef, Old High German ibu, German ob, Dutch of "if, whether"), of uncertain origin or relation. Perhaps from PIE pronominal stem *i- [Watkins]; but Klein, OED suggest it probably originally from an oblique case of a noun meaning "doubt" (compare Old High German iba "condition, stipulation, doubt," Old Norse if "doubt, hesitation," Swedish jäf "exception, challenge"). As a noun from 1510s.
iffy (adj.): 1937, American English, from if + -y (2). Originally associated with President Franklin D. Roosevelt.
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.
Ignatius: masc. proper name, from Latin Ignatius, collateral form of Egnatius. St. Ignatius, bishop of Antioch, was one of the apostolic fathers, martyred under Trajan; a set of epistles was attributed to him. Ignatius Loyola (1491-1556) was the founder of the Jesuits. Related: Ignatian.
igneo-: word-forming element meaning "of fire; of fire and; of igneous origin," from Latin igneus (see igneous).
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].
ignitable (adj.): 1640s; see ignite + -able. Ignite (adj.) "glowing with fire" is recorded from 1550s.
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.
ignition (n.): 1610s, "act of heating to the point of combustion," from French ignition or directly from Medieval Latin ignitionem (nominative ignitio), noun of action from past participle stem of Latin ignire "set on fire," from ignis "fire" (see igneous). Meaning "means of sparking a fire" (originally in a gun) is from 1881; meaning "means of sparking an internal combustion engine" is from 1906.
ignivomous (adj.): "vomiting fire," c. 1600, from Late Latin ignivomous, from Latin ignis "fire" (see igneous) + vomere "to vomit" (see vomit (n.)).
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.
ignominy (n.): 1530s, back-formation from ignominious or else from Middle French ignominie (15c.), from Latin ignominia "disgrace, dishonor" (see ignominious). Also sometimes shortened to ignomy.
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.
ignorance (n.): c. 1200, "lack of wisdom or knowledge," from Old French ignorance (12c.), from Latin ignorantia "want of knowledge" (see ignorant). Ignoration (1832) has been used in the sense "act of ignoring."
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.