Online Etymology Dictionary

insightful (adj.): "with penetrating understanding into character or hidden nature," 1881, from insight + -ful. Some earlier words in the same sense were insighted (c. 1600), inseeing "having insight" (1590s). Related: Insightfully; insightfulness.
insanity (n.): 1580s, "state of being insane, seriously impaired state of mental functioning," from Latin insanitatem (nominative insanitas) "unhealthfulness, unsoundness, disease," noun of quality from insanus "mad, insane; outrageous, excessive" (see insane). Meaning "extreme folly" is from 1844. The Latin abstract noun was insania ""unsoundness of mind, madness, frenzy."
inquiry (n.): early 15c., enquery, "a judicial examination of facts to determine truth;" mid-15c. in general sense "attempt to learn something, act or fact of inquiring," probably an Anglo-French noun developed from enqueren "to inquire" (see inquire). Respelled from mid-16c. to conform to Latin.
innovative (adj.): 1796 (with an isolated use from c. 1600), from innovate + -ive. Related: Innovatively; innovativeness.
innovation (n.): mid-15c., "restoration, renewal," from Late Latin innovationem (nominative innovatio), noun of action from past participle stem of innovare "to change; to renew" (see innovate). Meaning "a novel change, experimental variation, new thing introduced in an established arrangement" is from 1540s.
initiate (n.): "one who has been initiated" (in secret doctrines, etc.), 1732, from obsolete or archaic past participle adjective initiate "initiated, instructed in secret knowledge" (c. 1600), from Latin initiatus (see initiate (v.)).
initiate (v.): c. 1600, "introduce to some practice or system," also "begin, set going," from Late Latin initiatus, past participle of initiare "to begin, originate," in classical Latin only in the sense "to instruct in mysteries or sacred knowledge." This is from initium "a beginning; an entrance," also in plural initia "constituent parts; sacred mysteries," a noun use of the neuter past participle of inire "to go into, enter upon, begin," from in- "into, in" (see in- (2)) + ire "to go" (see ion).

In some senses the English word is a back-formation from initiation. Related: Initiated; initiates; initiating; initiator.
injure (v.): mid-15c., "do an injustice to, dishonor," probably a back-formation from injury, or else from Old French injuriier "to damage; offend," from Latin iniuriari "do an injury," from iniuria. Injury itself also served as a verb meaning "to injure, hurt, harm" (late 15c.). Related: Injured; injuring.
institutionalize (v.): "to put into institutional life" (usually deprecatory), 1897; see institution. Earlier (1860) it meant "to make into an institution" and "to adjust to life in an institution" (1893). Related: Institutionalized.
intelligentsia (n.): "the intellectual class collectively," 1905, from Russian intelligyentsiya, from Latin intelligentia "intelligence" (see intelligence). Perhaps via Italian intelligenzia.
interesting (adj.): 1711, "that concerns, important" (archaic), present-participle adjective from interest (v.). Meaning "engaging the attention, so as to excite interest" is from 1751. Related: Interestingly. Euphemistic phrase interesting condition, etc., "pregnant" is from 1748.
interdependence (n.): 1816 (Coleridge), from inter- + dependence.
inter alia: Latin, literally "amongst other things." from inter "among, between" (see inter-) + alia, neuter accusative plural of alius "(an)other" (see alias (adv.)). Latin for "among other persons" is inter alios.
intelligent (adj.): c. 1500, a back-formation from intelligence or else from Latin intelligentem (nominative intelligens), present participle of intelligere. Intelligent design, as a name for an alternative to atheistic cosmology and the theory of evolution, is from 1999. Related: Intelligently.
intellectual (adj.): late 14c., "grasped by the understanding" (rather than by the senses), from Old French intellectuel (13c.) and directly from Latin intellectualis "relating to the understanding," from intellectus "discernment, understanding," noun use of past participle of intelligere "to understand, discern" (see intelligence).

Sense of "characterized by a high degree of intellect" is from 1819. Meaning "appealing to or engaging the mental powers" is from 1834. Intellectual property "products of the intellect" is attested from 1845. Adjective formations in the sense "of or pertaining to the intellect" included intellective (early 15c.), intellectile (1670s).
integration (n.): 1610s, "act of bringing together the parts of a whole," from French intégration and directly from Late Latin integrationem (nominative integratio) "renewal, restoration," noun of action from past participle stem of Latin integrare "make whole," also "renew, begin again" (see integrate). Anti-discrimination sense (opposed to segregation) is recorded from 1934.
integrated (adj.): 1580s, "combined into a whole," past participle adjective from integrate (v.). Sense of "desegregated, not or no longer divided by race, etc." is from 1947.
insubordination (n.): 1790, on the model of French insubordination (1775); from in- (1) "not, opposite of" + subordination.
instigate (v.): 1540s, back-formation from instigation or else from Latin instigatus, past participle of instigare "to urge on, incite" (source also of French instiguer). Related: Instigated; instigates; instigating.
interpose (v.): 1590s (trans.); 1610s (intrans.), from Middle French interposer (14c.), from inter- "between" (see inter-) + poser "to place" (see pose (v.1)). Related: Interposed; interposing.
interpretation (n.): mid-14c. "a translated text, a translation" (late 13c. in Anglo-French), from Old French interpretacion, entrepretatiun "explanation, translation" (12c.) and directly from Latin interpretationem (nominative interpretatio) "explanation, exposition," noun of action from past participle stem of interpretari "explain, expound; understand" (see interpret).

From late 14c. as "act or process of explaining or interpreting; an explanation; construction placed upon an action." Meaning "dramatic or musical representation" is from 1880.
interpreter (n.): "one who translates spoken languages; a translator of written texts," late 14c., from Old French interpreteor, entrepreteur, from Late Latin interpretator "an explainer," agent noun from interpretari "explain, expound" (see interpret).
interpretative (adj.): "meant to explain," 1560s, from past participle stem of Latin interpretari "explain, expound; understand" (see interpret). Interpretive means the same but is considered to be less correctly formed, because -ive adjectives are normally formed on the Latin past participle. Related: Interpretatively.
interpretive (adj.): 1670s, from interpret + -ive, perhaps on model of assertive or other like words, where the -t- belongs to the Latin stem. The preferred formation is interpretative. Listed by Fowler among the words "that for one reason or another should not have been brought into existence."
intersect (v.): 1610s (trans.), back-formation from intersection, or else from Latin intersectus, past participle of intersecare "intersect, cut asunder," from inter- "between" (see inter-) + secare "to cut" (see section (n.)). Intransitive sense is from 1847. Related: Intersected; intersecting.
intimation (n.): mid-15c., "action of making known," from Middle French intimation (14c.), from Late Latin intimationem (nominative intimatio) "an announcement," noun of action from past participle stem of Latin intimare "make known, announce, impress" (see intimate (adj.)). Meaning "action of expressing by suggestion or hint, indirect imparting of information" is from 1530s.
intoxication (n.): c. 1400, intoxigacion "poisoning, administration of poison," from Medieval Latin intoxicationem (nominative intoxicatio) "a poisoning," noun of action from past participle stem of intoxicare "to poison" (see intoxicate). Meaning "state of inebriation" is from 1640s.
intoxicant (n.): "that which intoxicates," 1798; see intoxicate. Perhaps from Medieval Latin intoxicantem (nominative intoxicans), present participle of intoxicare. As an adjective from 1882.
introduce (v.): early 15c., "convey or bring (something) in or into," a back-formation from introduction or else from Latin introducere "to lead in, bring in." Meaning "to bring forward, open to notice" (of a subject, etc.) is from 1550s. Sense of "bring into personal acquaintance, make known" (as of one person to another) is from 1650s. Related: Introduced; introducing.
introspective (adj.): "having the quality of looking within," 1820 (Southey), from Latin introspect-, past participle stem of introspicere "look into, look at" (see introspection) + -ive. Related: Introspectively; introspectiveness.
introverted (adj.): 1781, "directed inward" (of the mind, etc.), past participle adjective from introvert (v.). Psychological sense is from 1915. Other adjectives in the non-psychological sense were introversive (1820), introvertive (1846), introverse (1874).
intimacy (n.): 1640s, from intimate (adj.) + -cy. Sense of "sexual intercourse" attested from 1670s but modern use is from newspaper euphemistic use (1882).
intervene (v.): 1580s, "intercept" (obsolete), a back-formation from intervention, or else from Latin intervenire "to come between, intervene; interrupt; stand in the way, oppose, hinder," from inter "between" (see inter-) + venire "to come" (see venue). Sense of "come between, fall or happen between" (of events) is from c. 1600; that of "interfere, interpose oneself between, act mediatorially" is from 1640s. Related: Intervened; intervener; intervening.
intuitive (adj.): 1640s, "perceiving directly and immediately," from Middle French intuitif or directly from Medieval Latin intuitivus, from intuit-, past participle stem of Latin intueri "look at, consider," from in- "into" (see in- (2)) + tueri "to look at, watch over" (see tutor (n.)). Meaning "self-evident" is from 1833. Related: Intuitively; intuitiveness.
inundate (v.): 1620s, back-formation from inundation, or else from Latin inundatus, past participle of inundare "to overflow, run over" (source also of Spanish inundar, French inonder). Related: Inundated; inundating.
invite (v.): "solicit to come," 1530s, a back-formation from invitation, or else from Middle French inviter (15c.), from Latin invitare "to invite," also "to summon, challenge; to feast, to entertain," a word of uncertain origin. Related: Invited; inviting.
ironside: name given to a man of great hardihood or bravery, c. 1300, first applied to Edmund II, king of England (d.1016), later also to Oliver Cromwell and his troops. Old Ironsides as a nickname of U.S.S. "Constitution" dates from that ship's defeat of H.M.S. "Guerriere" on Aug. 19, 1812, in the War of 1812.
ironic (adj.): 1620s, "pertaining to irony," from Late Latin ironicus, from Greek eironikos "dissembling, putting on a feigned ignorance," from eironeia (see irony). Related: Ironical (1570s); ironically.
irrelevant (adj.): 1680s, from assimilated form of in- (1) "not, opposite of" + relevant. Related: Irrelevantly.
It is worth remembering that irrelevant & relieving are the same word; that, presumably, is irrelevant which does not relieve or assist the problem in hand by throwing any light upon it. [Fowler]
irresistible (adj.): 1590s, from Late Latin irresistibilis, from assimilated form of in- "not, opposite of" (see in- (1)) + resistere (see resist). Related: Irresistibly; irresistibility.
irreverent (adj.): "deficient in veneration or respect," mid-15c., from Old French irreverent or directly from Latin irreverentem "disrespectful, irreverent" (see irreverence). Related: Irreverently (early 15c.); irreverential. Irreverend (late 15c.) means "not worthy of respect or veneration."
isolationist (n.): 1899 in reference to U.S. foreign policy, "one who advocates a policy of non-participation in foreign affairs" (earlier in reference to treatment of leprosy), from isolation + -ist. As an adjective from 1920. Isolationism is attested in a general sense by 1902; in a U.S. geopolitical sense from 1922.
isolation (n.): 1800, noun of action from isolate (v.) or else from French isolation, noun of action from isoler (see isolated).
itinerary (n.): mid-15c., "route of travel," from Late Latin itinerarium "account of a journey, description of a route of travel, road-book," noun use of neuter of itinerarius "of a journey," from Latin itineris "a journey," from ire "go" (see ion). By early 15c. it meant "record of a journey;" extended sense "sketch of a proposed route, list of places to be included in a journey" is from 1856.
illusory (adj.): 1590s, from French illusorie, from Late Latin illusorius "ironical, of a mocking character," from illus-, past participle stem of Latin illudere "mock, jeer at, make fun of," literally "play with," from assimilated form of in- "at, upon" (see in- (2)) + ludere "to play" (see ludicrous).
illusive (adj.): "deceptive, false, illusory," 1670s, from stem of illusion + -ive. The older adjective is illusory.
illusionist (n.): "conjurer, magic act performer," 1840, from illusion + -ist. Earlier "one suffering from illusions" (1812). Middle English had illusor "deceiver, deluder."
influential (adj.): "powerful, having influence," 1650s, from Medieval Latin influentialis, from influentia (see influence (n.)). Earlier in an astrological sense (1560s). Related: Influentially.
inflammable (adj.): "able to be set alight," c. 1600, from Middle French inflammable, from Medieval Latin inflammabilis, from Latin inflammare "to set on fire" (see inflame).Since 1980s use of the word, especially in safety warnings, has been sometimes discouraged for fear it could be misunderstood as meaning "non-flammable" through confusion of the two prefixes in-. The word was used earlier in medicine in the sense "liable to inflammation" (early 15c.). Related: Inflammability.
inflammation (n.): early 15c., in pathology, "excessive redness or swelling in a body part," from Old French inflammation (14c.) and directly from Latin inflammationem (nominative inflammatio) "a kindling, a setting on fire," noun of action from past participle stem of inflammare "to set on fire" (see inflame). Literal sense "act of setting on fire" in English is from 1560s.