Online Etymology Dictionary

interplay (n.): 1838, from inter- "between" + play (n.). "Reciprocal play," thus "free interaction."
Interpol: 1952, contraction of international police (in full, The International Criminal Police Commission), founded 1923 with headquarters in Paris.
interpolate (v.): 1610s, "to alter or enlarge (a writing) by inserting new material," from Latin interpolatus, past participle of interpolare "alter, freshen up, polish;" of writing, "falsify," from inter- "among, between" (see inter-) + polare, which is related to polire "to smoothe, polish."

Sense evolved in Latin from "refurbish," to "alter appearance of," to "falsify (especially by adding new material)." Middle English had interpolen (early 15c.) in a similar sense. Related: Interpolated; interpolating.
interposition (n.): late 14c., from Old French interposicion "interpolation, intercalation; suspension, break" (12c.), from Latin interpositionem (nominative interpositio) "an insertion," noun of action from past participle stem of interponere "to put between, place among; put forward," from inter- "between" (see inter-) + ponere "to put, place" (past participle positus; see position (n.)).
interpret (v.): late 14c., "expound the meaning of, render clear or explicit," from Old French interpreter "explain; translate" (13c.) and directly from Latin interpretari "explain, expound, understand," from interpres "agent, translator," from inter- "between" (see inter-) + second element of uncertain origin, perhaps related to Sanskrit prath- "to spread abroad," PIE *per- (5) "to traffic in, sell" (see pornography). Related: Interpreted; interpreting.
interregnum (n.): 1580s, from Latin interregnum "an interval between two reigns," literally "between-reign," from inter- "between" (see inter-) + regnum "kingship, dominion, rule, realm," related to regere "to rule, to direct, keep straight, guide" (see regal). In the republic, it meant a vacancy in the consulate. The earlier English noun was interreign (1530s), from French interrègne (14c.).
interrogation (n.): late 14c., "a question;" c. 1500, "a questioning; a set of questions," from Old French interrogacion "a questioning" (13c.) or directly from Latin interrogationem (nominative interrogatio) "a question; questioning; judicial inquiry," noun of action from past participle stem of interrogare "to ask, question, inquire; interrogate judicially, cross-examine," from inter- "between" (see inter-) + rogare "to ask, to question" (see rogation).
interrupt (v.): c. 1400, "to interfere with a legal right," from Latin interruptus, past participle of interrumpere "break apart, break off, break through," from inter- "between" (see inter-) + rumpere "to break" (see rupture (n.), and compare corrupt (adj.)). Meaning "to break into, break in upon, disturb the action of" (especially of speech) is from early 15c. in English (it is also in Latin). Related: Interrupted; interrupting.
intersection (n.): 1550s, "act or fact of crossing," from Middle French intersection (14c.) and directly from Latin intersectionem (nominative intersectio) "a cutting asunder, intersection," noun of action from past participle stem of intersecare "intersect, cut asunder," from inter- "between" (see inter-) + secare "to cut" (see section (n.)). In English originally a term in geometry; meaning "crossroads, a place of crossing" is from 1864. Related: Intersectional.
intersperse (v.): 1560s, from Latin interspersus "strewn, scattered, sprinkled upon," past participle of *interspergere, from inter- "between" (see inter-) + spargere "to scatter" (see sparse). Related: Interspersed; interspersing.
interstate (adj.): 1838, American English, in reference to traffic in slaves, from inter- "between" + state (n.) in the U.S. sense. Interstate commerce is that carried on by persons in one U.S. state with persons in another. Noun sense of "an interstate highway" is attested by 1975, American English.
interstellar (adj.): 1620s, "situated between or among the stars," in reference to the night sky (modern astronomical sense is from 1670s), from inter- "between" + Latin stella "star" (see stellar). Intersiderial in the same sense is from 1650s.
interstice (n.): early 15c., from Old French interstice (14c.) and directly from Latin interstitium "interval," literally "space between," from inter- "between" (see inter-) + stem of stare "to stand," from PIE root *stā- "to stand, set down, make or be firm" (see stet). Related: Interstices.
interval (n.): early 14c., "time elapsed between two actions or events," from Old French intervalle "interval, interim" (14c.), earlier entreval (13c.) and directly from Late Latin intervallum "a space between, an interval of time, a distance," originally "space between palisades or ramparts" [OED], from inter "between" (see inter-) + vallum "rampart, palisade, wall," which is apparently a collective form of vallus "stake," from PIE *walso- "a post" (see wall (n.)).

Metaphoric sense of "gap in time" also was in Latin. From c. 1400 in English as "a pause, an interruption in a state or activity." Musical sense "difference in pitch between two tones" is from c. 1600. Related: Intervallic.
intervention (n.): early 15c., "intercession, intercessory prayer," from Middle French intervention or directly from Late Latin interventionem (nominative interventio) "an interposing, a giving security," literally "a coming between," noun of action from past participle stem of Latin intervenire "to come between, interrupt," from inter- "between" (see inter-) + venire "come" (see venue). Later "act of intervening" in any way; in 19c.-20c. often of international relations; by 1983 of interpersonal intrusions by friends or family meant to reform a life felt to be going wrong.
interview (n.): 1510s, "face-to-face meeting, formal conference," from Middle French entrevue, verbal noun from s'entrevoir "to see each other, visit each other briefly, have a glimpse of," from entre- "between" (see inter-) + Old French voir "to see" (from Latin videre; see vision). Modern French interview is from English. Journalistic sense "conversation with someone to obtain statements for publication" is from 1869 in American English.
The 'interview,' as at present managed, is generally the joint product of some humbug of a hack politician and another humbug of a newspaper reporter. ["The Nation," Jan. 28, 1869]
Meaning "personal meeting to discuss hiring or employment" is by 1921; earlier it was used in military recruiting (1918).
terra incognita (n.): "unknown or unexplored region," 1610s, Latin, literally "unknown land," from terra (see terrain) + fem. of incognito.
over-indulgence (n.): also overindulgence, 1630s, from over- + indulgence. First attested in Donne.
intractable (adj.): c. 1500, "rough, stormy;" 1540s, "not manageable," from French intractable (15c.) or directly from Latin intractabilis "not to be handled, unmanageable," from in- "not, opposite of" (see in- (1)) + tractabilis (see tractable). Related: Intractably.
intramural (adj.): 1846, "within the walls, being within the walls or boundaries" (of a city, building, etc.), from intra- "within" + Latin muralis "pertaining to a wall," from murus "wall" (see mural). Equivalent to Late Latin intramuranus. Originally in English in reference to burials of the dead; in reference to college activities from 1871 (first at Columbia).
intransigent (adj.): 1874, "uncompromising, refusing to agree or come to understanding," (used of extreme political factions or parties), from French intransigeant (18c.), from Spanish los intransigentes, literally "those not coming to agreement," name for extreme left in the Spanish Cortes and the extreme republicans of the 1870s, from in- "not" (see in- (1)) + transigente "compromising," from Latin transigentem (nominative transigens), present participle of transigere "come to an agreement, accomplish, to carry through" (see transaction). It acquired its generalized sense in French. As a noun in English from 1879.
intransitive (adj.): 1610s, from Late Latin intransitivus "not transitive, not passing over" (to another person), Priscian's term, from in- "not" (see in- (1)) + Latin transitivus "that may pass over," from transire "to pass over" (see transitive). The noun meaning "an intransitive verb" is attested from 1824.
intravenous (adj.): "in or occurring within a vein," 1847, from intra- "within, inside" + Latin venous, from vena "vein" (see vein). Related: Intravenously.
intrepid (adj.): "unmoved by danger, undaunted," 1690s, from French intrépide (16c.) and directly from Latin intrepidus "unshaken, undaunted, not alarmed," from in- "not" (see in- (1)) + trepidus "alarmed," from PIE *trep-(1) "to tremble" (see trepidation). Related: Intrepidly; intrepidness (1620s).
intricate (adj.): early 15c., from Latin intricatus "entangled," past participle of intricare "to entangle, perplex, embarrass," from in- "in" (see in- (2)) + tricae (plural) "perplexities, hindrances, toys, tricks," a word of uncertain origin (compare extricate). Related: Intricately; intricateness.
intrigue (v.): 1610s, "to trick, deceive, cheat," from French intriguer (16c.), from Italian intrigare "to plot, meddle; perplex, puzzle," from Latin intricare "to entangle, perplex, embarrass" (see intricate).

Meaning "to plot or scheme" first recorded 1714. That of "to excite curiosity" is from 1894 (OED calls this use "A modern gallicism"). It also could mean "carry on a clandestine or illicit sexual relationship" (1650s). The word appears earlier in English as entriken "entangle, ensnare; involve in perplexity, embarrass" (late 14c.), from Old French entrique or directly from the Latin verb. Related: Intrigued; intriguer; intriguing. Dutch intrigueren, German intriguiren are from French.
intrinsic (adj.): late 15c., "interior, inward, internal," from Middle French intrinsèque "inner" (14c.), from Medieval Latin intrinsecus "interior, internal," from Latin intrinsecus (adv.) "inwardly, on the inside," from intra "within" (see intra-) + secus "along, alongside," from PIE *sekw-os- "following," suffixed form of root *sekw- (1) "to follow" (see sequel).

The form in English was conformed to words in -ic by 18c. Meaning "belonging to the nature of a thing" is from 1640s. Related: Intrinsical; intrinsically.
intro-: word-forming element, from Latin intro (adv.) "in, on the inside, within, to the inside," from PIE *en-t(e)ro-, suffixed form of preposition *en "in" (see in).
introduction (n.): late 14c., "act of bringing into existence," from Old French introduccion (14c.) and directly from Latin introductionem (nominative introductio) "a leading in," noun of action from past participle stem of introducere "to lead in, bring in; introduce; found, establish; bring forward (as an assertion)," from intro- "inward, to the inside" (see intro-) + ducere "to lead" (see duke (n.)).

Meanings "initial instruction in a subject" and "an introductory statement" are from mid-15c.; meaning "elementary treatise on some subject" is from 1520s. The sense of "formal presentation of one person to another" is from 1711.
introit (n.): in liturgics, "an antiphon sung as the priest approaches the altar to celebrate mass," late 15c., from Old French introite "(liturgical) introit; entrance" (14c.), from Latin (antiphona ad) introitum, from introitus "a going in, an entering, entrance; a beginning, prelude," past participle of introire "to enter," from intro- "on the inside, within" (see intro-) + ire "to go" (see ion).
introspection (n.): 1670s, "action of closely inspecting or examining," noun of action from past participle stem of Latin introspicere "to look into, look at, examine, observe attentively," from intro- "inward" (see intro-) + specere "to look at" (see scope (n.1)). Meaning "action of searching one's feelings or thoughts" is from 1807.
introvert (v.): 1650s, from Latin intro "inward, within" (see intro-) + vertere "to turn" (see versus).
intrusion (n.): late 14c., "unjust invasion of property or usurpation of office," from Old French intrusion (14c.), from Medieval Latin intrusionem (nominative intrusio) "a thrusting in," noun of action from past participle stem of Latin intrudere "to thrust in, force in," from in- "in" (see in- (2)) + trudere "to thrust, push," from PIE *treud- "to press, push, squeeze" (see threat).

Meaning "a thrusting or pushing in" is from 1590s; that of "act of intruding" is from 1630s. Geological sense is from 1816.
intuition (n.): mid-15c., intuicioun, "insight, direct or immediate cognition, spiritual perception," originally theological, from Late Latin intuitionem (nominative intuitio) "a looking at, consideration," noun of action from past participle stem of Latin intueri "look at, consider," from in- "at, on" (see in- (2)) + tueri "to look at, watch over" (see tutor (n.)).
inundation (n.): early 15c., from Latin inundationem (nominative inundatio) "an overflowing," noun of action from past participle stem of inundare "to overflow," from in- "onto" (see in- (2)) + undare "to flow," from unda "a wave," from PIE *unda-, nasalized form of root *wed- (1) "water, wet" (see water (n.1)).
inure (v.): formerly also enure, mid-15c., "accustom, adapt, establish by use," contracted from phrase (put) in ure "in practice" (early 15c.), from obsolete noun ure "work, practice, exercise, use," probably from Old French uevre, oeuvre "work," from Latin opera "work" (see opus). Meaning "toughen or harden by experience" is from late 15c. Related: Inured; inuring.
invalid (adj.1): "not strong, infirm," also "infirm from sickness, disease, or injury", 1640s, from Latin invalidus "not strong, infirm, impotent, feeble, inadequate," from in- "not" (see in- (1)) + validus "strong" (see valid). With pronunciation from French invalide (16c.).
invaluable (adj.): 1570s, "above value, too valuable for exact estimate," from in- (1) "not" + value (v.) "estimate the worth of" + -able. It also has been used in a sense "without value, worthless" (1630s, from in- + valuable). Related: Invaluably.
invariable (adj.): early 15c., from Old French invariable (14c.) and directly from Medieval Latin invariabilis, from in- "not, opposite of" (see in- (1)) + variabilis "changeable" (see variable). Related: Invariably.
invasion (n.): mid-15c., from Old French invasion "invasion, attack, assaut" (12c.), from Late Latin invasionem (nominative invasio) "an attack, invasion," noun of action from past participle stem of Latin invadere "to go, come, or get into; enter violently, penetrate into as an enemy, assail, assault, make an attack on," from in- "in" (see in- (2)) + vadere "go, walk" (see vamoose).
invective (n.): "an attacking in words," 1520s, from Medieval Latin invectiva "abusive speech," from Late Latin invectivus "abusive, scolding" from invect-, past participle stem of invehere "to carry in, bring in, introduce; assault, assail; attack with words" (see inveigh). For nuances of usage, see humor (n.). The earlier noun form in English was inveccion (mid-15c.), and invective (adj.) was in Middle English.
inveigh (v.): formerly also enveigh, late 15c., "to introduce," from Latin invehere "to bring in, carry in, introduce," also "assault, assail," from in- "against" (see in- (1)) + vehere "to carry" (see vehicle). Meaning "to give vent to violent denunciation" is from 1520s, from a secondary sense in Latin (see invective). Related: Inveighed; inveighing.
inveigle (v.): formerly also enveigle, etc., late 15c., "to blind (someone's) judgment," apparently an alteration of Middle French aveugler "delude, make blind," from Vulgar Latin *aboculus "without sight, blind," from Latin ab- "without" (see ab-) + oculus "eye" (see eye (n.)). The Latin word is a loan-translation of Greek ap ommaton "without eyes." Meaning "to win over by deceit, seduce" is 1530s. Related: Inveigler; inveiglment.
invention (n.): early 15c., "finding or discovering of something," from Old French invencion (13c.) and directly from Latin inventionem (nominative inventio) "faculty of invention," noun of action from past participle stem of invenire "to come upon, find; find out; invent, discover, devise; ascertain; acquire, get earn," from in- "in, on" (see in- (2)) + venire "to come" (see venue). Sense of "thing invented" is first recorded 1510s; that of "act or process of finding out how to make or do" is from 1530s.
Invention is applied to the contrivance and production of something, often mechanical, that did not before exist, for the utilization of powers of nature long known or lately discovered by investigation. Discovery brings to light what existed before, but was not known. [Century Dictionary]
Earliest sense of the word in Middle English was "devised method of organization" (c. 1400), now obsolete. Meaning "finding or discovery of something" is preserved in Invention of the Cross, Church festival (May 3) celebrating the reputed finding of the Cross of the Crucifixion by Helena, mother of Constantine, in 326 C.E. The related classical Latin word for "a device, contrivance" was inventum.
inventory (n.): early 15c., from Old French inventoire "detailed list of goods, a catalogue" (15c.), from Medieval Latin inventorium, alteration of Late Latin inventarium "list of what is found," from Latin inventus, past participle of invenire "to find, discover, ascertain" (see invention). Altered in Medieval Latin by influence of words in -orium; it properly belongs with words in -ary. Related: Inventorial; inventorially.
inverse (adj.): in early use also enverse, mid-15c., from Latin inversus, past participle of invertere "turn about, turn upside-down, upset, reverse, invert" (see invert). Related: Inversely. As a noun, "inverted state or condition," 1680s, from the adjective.
invert (v.): "to turn (something) in an opposite direction; reverse the position, order, or sequence of," 1530s, from Middle French invertir or directly from Latin invertere "turn upside down, turn about; upset, reverse, transpose," figuratively "pervert, corrupt, misrepresent," of words, "to use ironically," from in- "in, on" (see in- (2)) + vertere "to turn" (see versus). Related: Inverted; inverting; invertedly.
invertebrate (adj.): "having naturally no backbone," 1819, from Latin in- "not" (see in- (1)) + vertebratus (Pliny), from vertebra "joint or articulation of the body, joint of the spine" (see vertebra). As a noun, "an invertebrate animal," 1826.
invest (v.): late 14c., "to clothe in the official robes of an office," from Latin investire "to clothe in, cover, surround," from in "in, into" (see in- (2)) + vestire "to dress, clothe," from PIE *wes-ti-, suffixed form of root *wes- (4) "to clothe" (see wear (v.)).

The meaning "use money to produce profit" first attested 1610s in connection with the East Indies trade, and is probably a borrowing of a special use of Italian investire (13c.) from the same Latin root, via the notion of giving one's capital a new form. Figurative sense of "to clothe (with attributes)" is from c. 1600. The military meaning "to besiege, surround with hostile intent" also is from c. 1600. Related: Invested; investing.
investigation (n.): early 15c., from Old French investigacion (14c.), from Latin investigationem (nominative investigatio) "a searching into, a searching for," noun of action from past participle stem of investigare "to trace out, search after," figuratively "search into, investigate," from in- "in, into" (see in- (2)) + vestigare "to track, trace," from vestigium "a footprint, a track" (see vestige).