Online Etymology Dictionary

idealism (n.): 1796, in the abstract sense, originally "belief that reality is made up only of ideas," from ideal (adj.) + -ism; on model of French idéalisme. Meaning "representing things in an ideal form" is from 1829.
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."
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.): 1829; see idealist + -ic.
idealization (n.): 1796; see idealize + -ation.
idealize (v.): 1786, 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, from idea + -ate (2). 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: Latin, literally "the same" (see identical).
idempotent: 1870, from Latin idem "the same" + potentem "powerful" (see potent).
identical (adj.): 1610s, as a term in logic, from Medieval Latin identicus "the same," from Late Latin identitas "identity, sameness," ultimately from comb. form of Latin idem "the same" (from id "it, that one;" see id) + demonstrative suffix -dem. General sense of "being the same or very similar" is from 1630s. 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," from French identification, probably from identifier (see identify). Sense of "becoming or feeling oneself one with another" is from 1857. Sense of "determination of identity" 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 "recognize" first recorded 1769. Meaning "make one (with), associate (oneself)" is from 1780. Sense of "serve as means of identification" is attested by 1886. Related: Identified; identifying.
identity (n.): c.1600, "sameness, oneness," from Middle French identité (14c.), from Late Latin (5c.) identitatem (nominative identitas) "sameness," from ident-, comb. form of Latin idem (neuter) "the same" (see identical); abstracted from identidem "over and over," from phrase idem et 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.
ideogram (n.): 1838, from comb. form of Greek idea (see idea) + -gram.
ideograph (n.): late 1830s, from comb. form of Greek idea (see idea) + -graph "instrument for recording; something written," from Greek graphe "writing," from graphein "to write, express by written characters," earlier "to draw, represent by lines drawn" (see -graphy). Related: Ideographic (1822); ideographical.
ideological (adj.): 1797, from ideology + -ical. 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) + -logy. Later used in a sense "impractical theorizing" (1813). Meaning "systematic set of ideas, doctrines" first recorded 1909.
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.): (plural) early 14c., "middle day of a Roman month," from Old French Ides (12c.), from Latin idus (plural), a word perhaps of Etruscan origin. 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 idio-, comb. form of idios "own, personal, private, one's own" (see idiom).
idiocrasy (n.): "peculiarity," 1680s, from Greek idiokrasia, from idio- (see idio-) + krasis "mixing, tempering."
idiocy (n.): 1520s, from idiot on model of prophecy, etc. Early alternatives included idiotacy (1580s), idiotry (1590s).
idiolect (n.): 1948, from idio- + second element abstracted from dialect.
idiom (n.): 1580s, "form of speech peculiar to a people or place," from Middle French idiome (16c.) and directly from Late Latin idioma "a peculiarity in language," from Greek idioma "peculiarity, peculiar phraseology," from idioumai "to appropriate to oneself," from idios "personal, private," properly "particular to oneself," 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" (cf. 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"). Meaning "phrase or expression peculiar to a language" is from 1620s.
idiomatic (adj.): 1712, from Latin idiomaticus, from Greek idiomatikos; from idios "one's own" (see idiom) + matos "thinking, animated" (see automaton).
idiopathy (n.): 1640s, Modern Latin, from Greek idiopatheia, from comb. form of idios "one's own" (see idiom) + -patheia, comb. form of pathos "suffering, disease, feeling" (see pathos). Related: idiopathic.
idiosyncrasy (n.): c.1600, from French idiosyncrasie, from Greek idiosynkrasia "a peculiar temperament," from idios "one's own" (see idiom) + synkrasis "temperament, mixture of personal characteristics," from syn "together" + krasis "mixture." Originally in English a medical term meaning "physical constitution of an individual." Mental sense first attested 1660s.
idiosyncratic (n.): 1779, 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).
Reader, suppose you were an idiot. And suppose you were a member of Congress. But I repeat myself. [Mark Twain, c.1882]
Idiot box "television set" is from 1959; idiot light "dashboard warning signal" is attested from 1968. Idiot savant attested by 1870.
idiotic (adj.): 1713, 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 (see idiot). Idiotical is from 1640s. Related: Idiotically.
idle (adj.): Old English idel "empty, void; vain; worthless, useless; not employed," common West Germanic (cf. Old Saxon idal, Old Frisian idel "empty, worthless," Old Dutch idil, Old High German ital, German eitel "vain, useless, mere, pure"), of unknown origin. Idle threats preserves original sense; meaning "lazy" is c.1300.
idle (v.): late 15c., "make vain or worthless," from idle (adj.). Meaning "spend or waste (time)" is from 1650s. Meaning "cause to be idle" is from 1789. Sense of "running slowly and steadily without transmitting power" (as a motor) first recorded 1916. Related: Idled; idling.
idleness (n.): Old English idelnes "frivolity, vanity, emptiness; vain existence;" see idle + -ness. Old English expressed the idea we attach to in vain by in idelnisse. Spenser, Scott, and others use idlesse to mean the same thing in a positive, pleasant sense.
idler (n.): 1530s, agent noun from idle.
idly (adv.): Old English idellice; see idle + -ly (2).
Ido: 1908, artificial language based on Esperanto, devised 1907; 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," from Late Latin idolum "image (mental or physical), form," used in Church Latin for "false god," from Greek eidolon "appearance, reflection in water or a mirror," later "mental image, apparition, phantom," also "material image, statue," from eidos "form" (see -oid). 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" 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" (see idolatry).
idolatrous (adj.): 1540s, from idolater + -ous.
idolatry (n.): mid-13c., from Old French idolatrie, from Vulgar Latin idolatria, shortened from Late Latin idololatria (Tertullian), from Ecclesiastical Greek eidololatria "worship of idols," from eidolon "image" (see idol) + latreia "worship, service" (see -latry).
idolize (v.): 1590s, from idol + -ize. Related: Idolized; idolizing.
idyll (n.): also idyl, c.1600, "picturesque pastoral poem," from 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" (late 18c. in sense "pertaining to an idyll"); from idyll + -ic.
if: Old English gif (initial g- in Old English pronounced with a sound close to Modern English -y-), from Proto-Germanic *ja-ba (cf. Old Saxon, Old Norse ef, Old Frisian gef, Old High German ibu, German ob, Dutch of "if, whether"), from PIE pronomial stem *i- [Watkins]; Klein, OED suggest probably originally from an oblique case of a noun meaning "doubt" (cf. 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.): 1824, Canadian English, from an Eskimo word for "house, dwelling" (cf. Greenlandic igdlo "house").
Ignatius: masc. proper name, from Latin Ignatius, collateral form of Egnatius.