Online Etymology Dictionary

inelastic (adj.): 1748, "not rebounding after a strain," from in- (1) "not, opposite of" + elastic (adj.). Figurative sense "rigid, unyielding" attested by 1867. Related: Inelasticity.
ineffective (adj.): 1650s, from in- (1) "not, opposite of" + effective. Related: Ineffectively; ineffectiveness (1744).
inexhaustible (adj.): c. 1600, from in- (1) "not, opposite of" + exhaustible (see exhaust (v.)). Perhaps modeled on French inexhaustible (15c.). Related: Inexhaustibly.
inexcusable (adj.): early 15c., from Latin inexcusabilis "without excuse; affording no excuse," from in- "not, opposite of" (see in- (1)) + excusabilis, from excusare "apologize, make an excuse for" (see excuse (v.)). Related: Inexcusably.
instinctual (adj.): 1841, from instinct (Latin instinctus) + -al (1). Related: Instinctually.
institutional (adj.): 1610s, "of or pertaining to an institution," from institution + -al (1). Related: Institutionally.
institutionalization (n.): 1898, noun of action from institutionalize (q.v.).
in-store (adj.): also instore, 1954, from in (prep.) + store (n.). In Middle English, instore was a verb meaning "to restore, renew," from Latin instaurare.
instructive (adj.): "serving to instruct or inform," 1610s, from instruct (v.) + -ive. An earlier adjective was instructing (1580s). Related: Instructively; instructiveness.
instrumental (adj.): late 14c., "of the nature of an instrument, serving as a means to an end," from Old French instrumental, from Medieval Latin *instrumentalis, from Latin instrumentum "a tool, apparatus" (see instrument (n.)). Meaning "serviceable, useful" is from c. 1600. Of music, c. 1500; noun meaning "musical composition for instruments only" is attested by 1940. Related: Instrumentally; instrumentality.
insupportable (adj.): 1520s, from Middle French insupportable (14c.) or directly from Late Latin insupportabilis, from in- "not" (see in- (1)) + *supportabilis, from Latin supportare "to carry" (see support (v.)). Related: Insupportably.
intensive (adj.): mid-15c., "intense, fervent, great," from Old French intensif (14c.) and Medieval Latin intensivus, from Latin intens-, past participle stem of intendere "turn one's attention; strain, stretch" (see intend).

Grammatical meaning "expressing intensity" is from c. 1600; as a noun, "something expressing intensity," 1813, from the adjective. Alternative intensitive is a malformation. Intensive care attested from 1958. Related: Intensively; intensiveness.
intention (n.): late 14c., entencioun, "purpose, design, aim or object; will, wish, desire, that which is intended," from Old French entencion "intent, purpose, aspiration; will; thought" (12c.), from Latin intentionem (nominative intentio) "a stretching out, straining, exertion, effort; attention," noun of action from intendere "to turn one's attention," literally "to stretch out" (see intend). Also in Middle English "emotion, feelings; heart, mind, mental faculties, understanding."
intentions (n.): "one's purposes with regard to courtship and marriage," by 1796; see intention.
intension (n.): c. 1600, "action of stretching; increase of degree or force," from Latin intensionem/intentionem (nominative intensio/intentio) "a stretching, straining," figuratively "exertion, effort," noun of action from past participle stem of intendere in its literal sense of "stretch out, strain" (see intend, and compare intention, which has the figurative sense). Related: Intensional.
intensification (n.): 1835, noun of action from intensify.
intendant (n.): "one who has charge of some business," 1650s, from French intendant (16c.), from Latin intendantem, present participle of intendere "turn one's attention, exert oneself" (see intend).
interconnect (v.): 1863, from inter- + connect (v.). Related: Interconnected; interconnecting.
intergalactic (adj.): 1928, in reference to galaxies as presently understood, from inter- + galactic. The word itself was in use by 1901, when galaxies were thought to be a sort of nebulae.
intergenerational (adj.): 1964, from inter- + generation + -al (1).
interlock (v.): 1630s, "to be locked together," from inter- "between" + lock (v.). Related: Interlocked; interlocking. As a noun, attested by 1856.
interdisciplinary (adj.): 1937, from inter- + disciplinary.
interdiction (n.): mid-15c., enterdiccioun, from Latin interdictionem (nominative interdictio) "a prohibiting, a forbidding," noun of action from past participle stem of interdicere (see interdict).
interdepartmental (adj.): also inter-departmental, 1861, from inter- + departmental.
intemperance (n.): early 15c., "lack of restraint, excess," also of weather, "inclemency, severity," from Old French intemperance (14c.) and directly from Latin intemperantia "intemperateness, immoderation, excess" (as in intemperantia vini "immoderate use of wine"), from in- "not, opposite of" (see in- (1)) + temperantia "moderation, sobriety, discretion, self-control," from temperans, present participle of temperare "to moderate" (see temper (v.)).
intentional (adj.): 1520s, from intention + -al (1) or else from Medieval Latin intentionalis. Intentional fallacy recorded from 1946. Related: Intentionality.
intentive (adj.): late 14c., "eager, assiduous; attentive, paying attention," from Old French ententif, intentif "attentive, solicitous, assiduous" (12c.), from Late Latin intentivus, from intent-, past participle stem of Latin intendere "turn one's attention" (see intend). Related: Intentively; intentiveness.
interactive (adj.): "acting upon or influencing each other," 1832, from interact (v.), probably on model of active. Related: Interactively; interactivity.
interbreed (v.): 1803 (transitive) "to breed by crossing species or varieties," from inter- + breed (v.). Intransitive sense of "procreate with one of a different species" is from 1825. Related: Interbred; interbreeding.
intercalary (adj.): "inserted into the calendar," 1610s, from Latin intercalarius "intercalary, of an intercalary month," from intercalare "proclaim an intercalary day" (see intercalate). General sense of "interpolated" is attested from 1798.
interception (n.): early 15c., "action of intercepting" (the flow of a bodily fluid), from Latin interceptionem (nominative interceptio) "a seizing, taking away," noun of action from past participle stem of intercipere (see intercept (v.)). Specific football/rugby sense is attested by 1897. Meaning "action of closing in on and destroying an enemy aircraft, etc." is recorded from 1939.
intercessor (n.): "one who pleads or intervenes on behalf of another," late 15c., from a specific Christian use of Latin intercessor "one who intervenes, a mediator," agent noun from intercedere (see intercede). Related: Intercessory.
interchangeable (adj.): late 14c., entrechaungeable, "mutual, reciprocal," from inter- + changeable. Meaning "capable of being used in place of each other" is from 1560s. Related: Interchangeably.
interference (n.): 1783, "intermeddling," from interfere on model of difference, etc. In physics, in reference to the mutual action of waves on each other, from 1802, coined in this sense by English scientist Dr. Thomas Young (1773-1829). Telephoning (later broadcasting) sense is from 1887. In chess from 1913; in U.S. football from 1894.
interlace (v.): formerly also enterlace, late 14c. (trans.), "unite by crossing the laces," thus, "entangle, bind together," from Old French entrelacier (12c.), from entre- (see entre-) "between" + lacier "to tie, entangle," from laz (see lace (n.)).

Intransitive sense from 1590s. Television sense is from 1927. Related: Interlaced; interlacing; interlacement. The noun is 1904, from the verb.
intellectually (adv.): late 14c., "to or by the understanding," from intellectual + -ly (2).
intercontinental (adj.): 1825, American English, from inter- "between" + continental (adj.). Of missiles, from 1956.
intermediary (adj.): 1757, "situated between two things;" 1818 as "serving as a mediator;" from French intermédiaire (17c.), from Latin intermedius "that which is between" (see intermediate). As a noun, "one who acts between others" from 1791 (Medieval Latin intermedium also was used as a noun). An earlier adjective was intermedial (1590s).
Intermediary, n., is, even with concrete sense of go-between or middleman or mediator, a word that should be viewed with suspicion & resorted to only when it is clear that every more ordinary word comes short of the need. [Fowler]
interment (n.): early 14c., from Old French enterrement "burial, interment," from enterrer (see inter (v.)).
intermingle (v.): late 15c. (trans.), from inter- "between" + mingle (v.). Intransitive sense from 1620s. Related: Intermingled; intermingling.
intermit (v.): 1540s, "to interrupt" (obsolete); 1570s as "to discontinue for a time, suspend" (trans.) and "cease for a time" (intrans.), from Latin intermittere "to leave off, leave an interval, omit, suspend, interrupt, neglect," from inter- "between" (see inter-) + mittere "to send" (see mission). Related: Intermitted; intermitting.
intermodal (adj.): 1963, from inter- "between" + modal (adj.).
internalization (n.): 1853, from internal + -ization.
internalize (v.): 1856, American English, from internal + -ize. Related: Internalized; internalizing.
internationalise (v.): chiefly British English spelling of internalize (q.v.). For spelling, see -ize. Related: Internationalisation.
internationalization (n.): 1860, with reference to law; see international + -ization.
interneuron (n.): 1939, from neuron + first element from internuncial (adj.) "communicating between different parts of the body," from Latin internuncius "a messenger, mediator," from inter- "between" (see inter-) + nuntius "messenger," from PIE root *neu- "to shout" (source also of Greek neuo "to nod, beckon," Old Irish noid "make known").
internist (n.): 1897, from internal (medicine) + -ist.
internment (n.): 1840, "confinement within a place," from intern (v.1) + -ment. Compare French internement. Internment camp is attested from 1916.
internship (n.): 1899, from intern (n.) + -ship.