Online Etymology Dictionary

ichthyology (n.): 1640s, from ichthyo- "fish" + -ology. Related: Ichthyologist; ichthyological.
ichthyomorphic (adj.): "fish-shaped," 1870 in biology, 1879 in mythology, from icthyo- "fish" + -morphic, from Greek morphe "form, shape" (see Morpheus).
ichthyophagous (adj.): "fish-eating," 1791, from Latinized form of ikhthyophagos "fish-eating;" see ichthyo- + -phagous. Related: Ichthyophagist (1727).
ichthyosaur (n.): extinct aquatic reptile, 1830, Modern Latin, from Latinized form of Greek ikhthys "fish" (see ichthyo-) + -saurus. Related: Ichthyosaurus (1819); ichthyosaurian.
ichthyosis (n.): 1815, coined in Modern Latin (1801); see ichthyo- + -osis. So called for the scales which form.
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.
icing (n.): 1769 in the confectionery sense, "coating of concreted sugar," verbal noun of ice (v.). Earlier in this sense was simple ice (1723); frosting came later. Meaning "process of becoming covered with ice" is from 1881.
ickle (adj.): childish pronunciation of little (adj.), attested by 1846.
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.
iconic (adj.): 1650s, "of or pertaining to a portrait," from Late Latin iconicus, from Greek eikonikos "pertaining to an image," from eikon "likeness, image, portrait" (see icon). In art, applied to statues of victorious athletes, sovereigns, etc., 1801.
iconoclasm (n.): 1797 in reference to an act of breaking or destroying idols physically; figuratively from 1858 in reference to beliefs, cherished institutions, etc.; see iconoclast. An older word for it was iconomachy (1580s), from Greek eikonomakhia (see -machy).
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.
iconoclastic (adj.): 1640s; see iconoclast + -ic. Related: Iconoclastically.
iconography (n.): 1670s, "illustration by drawing or figures," from Medieval Latin iconographia, from Greek eikonographia "sketch, description," from eikon (see icon) + -graphia (see -graphy). Related: Iconographic; iconographer.
iconology (n.): "study of icons," 1736; see icon + -logy.
icosahedron (n.): "twenty-sided body," 1560s, from Latinized form of Greek eikosahedron, noun use of neuter of eikosahedros, from eikosi "twenty" + -hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit" (see sedentary). Greek eikosi is from PIE *wikmti- "twenty," from *wi- "in half" (hence "two") + (d)kmti-, from root *dekm- "ten" (see ten). Related: icosahedral.
icteric (adj.): "jaundiced," c. 1600, from Latin ictericus, from Greek ikterikos "jaundiced," from ikteros "jaundice" (see icterus). Related: Icterical.
icterus (n.): "jaundice," 1706, medical Latin, from Greek ikteros "jaundice," also the name of a yellowish bird the sight of which was supposed, by sympathetic magic, to cure jaundice (but the bird died). As a zoological genus (American orioles), from 1713.
ictus (n.): rhythmical or metrical stress, 1752, from Latin ictus "a blow, stroke, thrust;" of voices "a beat, impulse, stress," from icere (past participle ictus) "to strike, hit," which is related to iacere "to throw" (see jet (v.)). Reckoned in Anglo-Saxon poetry; in Modern English it generally is identical to syllabic stress or accent.
icy (adj.): Old English isig; see ice (n.) + -y (2). Modern use is said to be a late Middle English re-formation. Figurative sense "characterized by coldness or chill, frigid" (of manners, expressions, etc.) is from 1590s. Similar formation in Dutch ijzig, German eisig, Swedish isig. Related: Icily; iciness.
id (n.): 1924, in Joan Riviere's translation of Freud's "Das Ich und das Es" (1923), from Latin id "it" (as a translation of German es "it" in Freud's title), used in psychoanalytical theory to denote the unconscious instinctual force. Latin id is from PIE pronominal stem *i- (see yon).
id est: Latin, literally "that is (to say)," from id "that," neuter of is, from PIE pronominal stem *i- (see yon). For est, see is. Usually abbreviated i.e. "to write, or even to say, this in the full instead of in the abbreviated form is now so unusual as to convict one of affectation" [Fowler]. It introduces another way to say something already said, not an example of it (which is e.g.).
Ida: fem. proper name, Medieval Latin, from Old High German Ida, which is perhaps related to Old Norse ið "work." As the name of a mountain near Troy and one in Crete, it probably is a different word, of unknown or non-IE origin; related: Idaean.
Idaho: 1861 as a place name, originally applied by U.S. Congress to a proposed territorial division centered in what is now eastern Colorado; said at the time to mean "Gem of the Mountains" but probably rather from Kiowa-Apache (Athabaskan) idaahe "enemy," a name applied by them to the Comanches. Modern Idaho was organized 1861 as a county in Washington Territory; in 1863 became a territory in its own right and it was admitted as a state in 1890.
ide (n.): see ides.
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].
ideal (n.): "(hypothetical) perfect person, thing, or state," 1796, in a translation of Kant, from ideal (adj.). Hence "standard or model of perfection" (1849).
idealism (n.): 1796 in the abstract metaphysical sense "belief that reality is made up only of ideas," from ideal (adj.) + -ism. Probably formed on model of French idéalisme. Meaning "tendency to represent things in an ideal form" is from 1829. Meaning "pursuit of the ideal, a striving after the perfect state" (of truth, purity, justice, etc.).

In the philosophical sense the Germans have refined it into absolute (Hegel), subjective (Fichte), objective (von Schelling), and transcendental (Kant).
idealist (n.): "one who represents things in an ideal form," 1829, from ideal + -ist. Earlier (1796) in a philosophical sense "one who believes reality consists only in (Platonic) ideals" (see idealism).
It seems even incredible, that any Idealist in any age could forget himself so far as to run his head against a post, merely because he found in his system, that no external world does exist, and that therefore nothing could be without to hurt him. [F.A. Nitsch, "A General and Introductory View of Professor Kant's Principles," 1796]
Earlier still, "one who holds doctrines of philosophical idealism" (1701).
idealistic (adj.): "striving after perfect good," 1819; see idealist + -ic. Related: Idealistically.
ideality (n.): 1817, "quality of being ideal;" see ideal (adj.) + -ity. In phrenology, "imaginative faculty" (1828); as the opposite of reality, 1877.
idealization (n.): 1796; see idealize + -ation. Perhaps via French idéalisation.
idealize (v.): 1786, "make ideal, consider as ideal," probably formed from ideal (adj.) + -ize. Related: Idealized; idealizing.
ideally (adv.): "in the best conceivable situation," 1840, from ideal + -ly (2). Earlier "in an archetype" (1640s); "in idea or imagination" (1590s).
ideate (v.): c. 1600, "imagine, form ideas," from idea + -ate (2). From 1862 as "to think." Related: Ideated; ideating.
ideation (n.): 1829; see idea + -ation. Related: Ideational.
As we say Sensation, we might say also, Ideation; it would be a very useful word; and there is no objection to it, except the pedantic habit of decrying a new term. [James Mill, "Analysis of the Phenomena of the Human Mind," London, 1829]
idem (adv.): "the same (as above)," used to avoid repetition in writing, Latin, literally "the same," from id "it, that one," from PIE pronominal stem *i- (see yon) + demonstrative suffix -dem.
idempotent (n.): in algebra, quantity which multiplied by itself gives itself, 1870, from Latin idem "the same, identical with" (see idem) + potentem "powerful" (see potent).
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.
identifiable (adj.): 1804, from identify + -able. Related: Identifiably.
identification (n.): 1640s, "treating of a thing as the same as another; act of making or proving to be the same," from French identification, probably from identifier (see identify). Psychological sense of "becoming or feeling oneself one with another" is from 1857. Meaning "act or process of determining the identity of something" is from 1859. Meaning "object or document which marks identity" is from 1947 (short for identification tag, card, etc.).
identifier (n.): "thing that identifies," 1870, agent noun from identify.
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.
ideo-: word-forming element variously used with reference to images or to ideas, from Greek idea (see idea).
ideogram (n.): "ideograph," 1837, from ideo-, here as a comb. form of idea, + -gram.
ideograph (n.): "character or symbol which suggests an object without expressing its name," 1841, from ideo-, here as a comb. form of idea, + -graph "instrument for recording; something written." Related: Ideographic (1822); ideographical.