Online Etymology Dictionary

intrigue (n.): 1640s, "a clandestine plot;" 1660s, "secret plotting," probably from intrigue (v.). Also used from 1660s as "clandestine or illicit sexual encounter."
intriguing (adj.): 1680s, "plotting, scheming," present-participle adjective from intrigue (v.). Meaning "exciting curiosity" is from 1909. Related: Intriguingly.
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 (n.): short for introduction, attested from 1923.
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).
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.
introducer (n.): 1620s, agent noun from introduce (v.).
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.
introductory (adj.): c. 1600, from Late Latin introductorius, from introduct-, past participle stem of Latin introducere "to lead in, bring in" (see introduction). Also used in English from c. 1400 as a noun meaning "introductory treatise or textbook."
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).
introject (v.): 1902 in psychology, probably a back-formation from introjection. Related: Introjected; introjecting.
intron (n.): 1978 in genetics, from intragenic "occurring within a gene" + -on.
introrse (adj.): "turned or facing inward," 1831 (earlier in French), from Latin introrsus (adv.) "toward the inside," a contraction of introversus, from intro "within" (see intro-) + versus "turned" (see versus).
introspect (v.): 1680s, "to look into" (transitive), from Latin introspectus, past participle of introspicere "look at, look into; examine, observe attentively," from intro- "inward" (see intro-) + specere "to look at" (see scope (n.1)). Meaning "look within, search one's feelings or thoughts" is from 1875, a back-formation from introspection. Related: Introspected; introspecting.
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.
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.
introversion (n.): 1650s, "action of turning inward" (of thought or contemplation), from Modern Latin introversionem, noun of action from past participle stem of *introvertere (see introvert (v.)). Psychological meaning "tendency to withdraw from the world" is from 1912.
introvert (v.): 1650s, from Latin intro "inward, within" (see intro-) + vertere "to turn" (see versus).
introvert (n.): 1878, in zoology, "part or organ which is turned in upon itself," from introvert (v.). The psychological sense "introverted person" (opposed to extrovert) is 1917, from German, introduced there by C.G. Jung (1875-1961).
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).
intrude (v.): early 15c., in an ecclesiastical sense, "take possession of (a prebend) not rightfully one's own," a back-formation from intrusion, or else from 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).

From 1560s in a physical sense of "thrust in" (transitive or intransitive); meaning "enter unbidden and without welcome" is from 1570s; that of "thrust or bring in without necessity or right" is from 1580s. Related: Intruded; intruding.
intruder (n.): 1530s, agent noun from intrude. Originally legal. Fuller ("Pisgah-Sight of Palestine," 1650) has fem. form intrudress.
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.
intrusive (adj.): c. 1400, "usurping," from Latin intrus-, past participle stem of intrudere (see intrusion) + -ive. Meaning "coming unbidden" is from 1640s. Geological sense "thrust in out of regular place" is from 1826. Related: Intrusively; intrusiveness.
intrust (v.): obsolete form of entrust (q.v.). Related: Intrusted; intrusting.
intubate (v.): 1610s, "to form into tubes," from in- (2) "in" + Latin tuba "tube" (see tuba) + -ate (2). Medical sense is from 1887. Related: Intubated. Intubation "act of inserting a tube" (into an orifice) is from 1885.
intuit (v.): 1776, "to tutor," from Latin intuit-, past participle stem of intueri "look at, consider," from in- "at, on" (see in- (2)) + tueri "to look at, watch over" (see tutor (n.)). Meaning "to perceive directly without reasoning, know by immediate perception" is from 1840 (De Quincey), in this sense perhaps a back-formation from intuition. Related: Intuited; intuiting.
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.)).
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.
intumescence (n.): "swollen state, expansion," 1650s, from French intumescence (17c.), from Latin intumescere "to swell up, rise, be elevated," of sounds, "grow louder," figuratively, "grow excited, become enraged," from in- "in" (see in- (2)) + tumescere "begin to swell, swell up" (see tumescence).
intumescent (adj.): "swelling up," 1796, from Latin intumescentem (nominative intumescens), present participle of intumescere "to swell up, rise, be elevated," of sounds, "grow louder," figuratively, "grow excited, become enraged," from in- "in" (see in- (2)) + tumescere "begin to swell, swell up" (see tumescence).
inturn (n.): 1590s, "turning in of the toes" (especially in dancing), from in + turn. In wrestling, "a lifting with the thigh" (c. 1600).
inturned (adj.): "turned inward," 1843, from in (adv.) + past participle of turn (v.).
intussusception (n.): "reception of one part within another," 1707, literally "a taking in," from Latin intus "within" (see ento-) + susceptionem (nominative susceptio) "a taking up, a taking in hand, undertaking," noun of action from past participle stem of suscipere "to take, catch, take up, lift up" (see susceptible).
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.
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)).
inurbane (adj.): c. 1600, from Latin inurbanus "not civil or polite," from in- "not" (see in- (1) + urbanus "refined, courteous," literally "of a city" (see urban (adj.)). Related: Inurbanity.
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.
inutile (adj.): late 15c., "unprofitable, useless," from French inutile (12c., inutele), from Latin inutilis "useless, unprofitable," from in- "not" (see in- (1)) + utilis "useful, beneficial, profitable," from uti "make use of, profit by, take advantage of" (see use (v.)). It appears to have fallen from use by 1700; modern use (from mid-19c.) is perhaps a reborrowing from French.
inutility (n.): 1590s, from Middle French inutilité (15c.), from Latin inutilitas "uselessness," from inutilis "useless, unprofitable," from in- "not" (see in- (1)) + utilis "useful, beneficial, profitable," from uti "make use of, profit by, take advantage of" (see use (v.)).
invade (v.): "enter in a hostile manner," late 15c., from 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). Compare evade. Related: invaded; invading.
invader (n.): 1540s, agent noun from invade.
invaginate (v.): "put into a sheath," 1650s, from Latin in- "in" (see in- (2)) + verb from vagina "a sheath" (see vagina). Related: Invaginated; invagination.
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.).
invalid (n.): "infirm or sickly person," 1709, originally of disabled military men, from invalid (adj.1). In Paris, Invalides is short for Hôtel des Invalides, home for old and disabled soldiers in the 7th arrondissement of Paris.
invalid (adj.2): "of no legal force," 1630s, from special use of Latin invalidus "not strong, weak, feeble" (see invalid (adj.1)).
invalidate (v.): 1640s, from invalid (adj.2) + -ate (2). Related: Invalidated; invalidating.
invalidation (n.): 1752, noun of action from invalidate (v.). Perhaps modeled on French invalidation (17c.).
invalidity (n.): 1540s, from Middle French invalidité (16c.) or directly from Medieval Latin invaliditas "weakness, infirmity," from Latin invalidus "not strong, weak, feeble" (see invalid (adj.1)).
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.