introspect (v.) Look up introspect at Dictionary.com
1680s, from Latin introspectus, past participle of introspicere "look at, look into" (see introspection). Related: Introspected; introspecting.
introspection (n.) Look up introspection at Dictionary.com
1670s, noun of action from past participle stem of Latin introspicere "to look into, look at," from intro- "inward" (see intro-) + specere "to look at" (see scope (n.1)).
introspective (adj.) Look up introspective at Dictionary.com
1820, from Latin introspect-, past participle stem of introspicere (see introspection) + -ive.
introversion (n.) Look up introversion at Dictionary.com
1650s, of thought or contemplation, from Modern Latin introversionem, noun of action from past participle stem of *introvertere (see introvert). Meaning "tendency to withdraw from the world" is from 1912.
introvert (v.) Look up introvert at Dictionary.com
1650s, from Latin intro- "inward" (see intro-) + vertere "to turn" (see versus). The noun, "introverted person" (opposed to extrovert) is 1918, from German psychology, introduced there by C.G. Jung (1875-1961).
introverted (adj.) Look up introverted at Dictionary.com
1781, "directed inward," past participle adjective from introvert. Psychological sense is from 1915.
intrude (v.) Look up intrude at Dictionary.com
early 15c., back-formation from intrusion, or else from Latin intrudere "to thrust in" (see intrusion). Related: Intruded; intruding.
intruder (n.) Look up intruder at Dictionary.com
1530s, agent noun from intrude. Originally legal.
intrusion (n.) Look up intrusion at Dictionary.com
late 14c., from Old French intrusion (14c.), from Medieval Latin intrusionem (nominative intrusio) "a thrusting in," noun of action from past participle stem of Latin intrudere, from in- "in" (see in- (2)) + trudere "to thrust, push" (see extrusion).
intrusive (adj.) Look up intrusive at Dictionary.com
c.1400, from Latin intrus-, past participle stem of intrudere (see intrusion) + -ive. Related: Intrusively; intrusiveness.
intubate (v.) Look up intubate at Dictionary.com
1610s, "to form into tubes," from in- (2) "in" + Latin tuba "tube" (see tuba) + -ate (2). Medical sense is from 1889. Related: Intubated; intubation.
intuit (v.) Look up intuit at Dictionary.com
1776, "to tutor," from Latin intuit-, past participle stem of intueri (see intuition). Meaning "to perceive directly without reasoning" is from 1840, in this sense perhaps a back-formation from intuition. Related: Intuited; intuiting.
intuition (n.) Look up intuition at Dictionary.com
mid-15c., 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 tuition).
intuitive (adj.) Look up intuitive at Dictionary.com
1640s, from Middle French intuitif or directly from Medieval Latin intuitivus, from intuit-, past participle stem of intueri "look at, consider" (see intuition). Related: Intuitively; intuitiveness.
intumescence (n.) Look up intumescence at Dictionary.com
1650s, from French intumescence, from Latin intumescere (see intumescent).
intumescent (adj.) Look up intumescent at Dictionary.com
1796, from Latin intumescentem (nominative intumescens), present participle of intumescere "to swell up," from in- "in" (see in- (2)) + tumescere (see tumescence).
inturn (n.) Look up inturn at Dictionary.com
1590s, "turning in of the toes" (especially in dancing), from in + turn.
intussusception (n.) Look up intussusception at Dictionary.com
1707, literally "a taking in," from Latin intus "within" (see ento-) + susceptionem "a taking up" (see susceptible).
inundate (v.) Look up inundate at Dictionary.com
1620s, back-formation from inundation, or else from Latin inundatus, past participle of inundare "to overflow, run over" (see inundation). Related: Inundated; inundating.
inundation (n.) Look up inundation at Dictionary.com
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 "wave" (see water (n.1)).
inure (v.) Look up inure at Dictionary.com
early 15c., in ure "in practice," from obsolete ure "work, practice, exercise, use," probably from Old French uevre, oeuvre "work," from Latin opera (see opus). Related: Inured; inuring.
inutile (adj.) Look up inutile at Dictionary.com
late 15c., from French inutile (12c., inutele), from Latin inutilis "useless, unprofitable," from in- "not" (see in- (1)) + utilis (see utility).
inutility (n.) Look up inutility at Dictionary.com
1590s, from Middle French inutilité (15c.), from Latin inutilitatem (nominative inutilitas) "uselessness," from inutilis (see inutile).
invade (v.) Look up invade at Dictionary.com
late 15c., from Middle French invader "to invade," and directly from Latin invadere "to go into, enter upon; assail, assault, attack" (see invasion). Related: invaded; invading.
invader (n.) Look up invader at Dictionary.com
1540s, agent noun from invade.
invaginate (v.) Look up invaginate at Dictionary.com
1650s, from Medieval Latin invaginatus, past participle of invaginare "to put into a sheath," from Latin in- "in" (see in- (2)) + vagina "a sheath" (see vagina). Related: Invaginated; invagination.
invalid (adj.1) Look up invalid at Dictionary.com
"not strong, infirm," 1640s, from Latin invalidus "not strong, infirm, weak, feeble," from in- "not" (see in- (1)) + validus "strong" (see valid). Specific meaning "infirm from sickness, disease, or injury" is from 1640s.
invalid (adj.2) Look up invalid at Dictionary.com
"of no legal force," 1630s, from special use of Latin invalidus (see invalid (adj.1)).
invalid (n.) Look up invalid at Dictionary.com
1709, originally of disabled military men, from invalid (adj.1). Invalides is short for French Hôtel des Invalides, home for old and disabled soldiers in the 7th arrondissement of Paris.
invalidate (v.) Look up invalidate at Dictionary.com
1640s, from invalid + -ate (2). Related: Invalidated; invalidating.
invalidation (n.) Look up invalidation at Dictionary.com
1771, noun of action from invalidate (v.).
invalidity (n.) Look up invalidity at Dictionary.com
1540s, from Latin invalidatus (see invalid (adj.)).
invaluable (adj.) Look up invaluable at Dictionary.com
1570s, "above value," from in- (1) "not" + value (v.) "estimate the worth of" + -able. It also has been used in a sense "without value, worthless" (1630s).
invariability (n.) Look up invariability at Dictionary.com
1640s, from invariable + -ity.
invariable (adj.) Look up invariable at Dictionary.com
early 15c., from Old French invariable (14c.) and directly from Medieval Latin invariabilis, from in- "not, opposite of" (see in- (1)) + variabilis (see variable). Related: Invariably.
invariant (adj.) Look up invariant at Dictionary.com
1851, from in- (1) "not" + variant.
invasion (n.) Look up invasion at Dictionary.com
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 "go into, fall upon, attack, invade," from in- "in" (see in- (2)) + vadere "go, walk" (see vamoose).
invasive (adj.) Look up invasive at Dictionary.com
mid-15c., from Middle French invasif (15c.), from Medieval Latin invasivus, from invas-, past participle stem of invadere (see invasion).
invective (n.) Look up invective at Dictionary.com
1520s, from Medieval Latin invectiva "abusive speech," from Late Latin invectivus "abusive," from Latin invectus, past participle of invehi "to attack with words" (see inveigh). For nuances of usage, see humor. The earlier noun form was inveccion (mid-15c.).
inveigh (v.) Look up inveigh at Dictionary.com
late 15c., "to introduce," from Latin invehi "to attack with words," originally "carry oneself against," passive infinitive of invehere "bring in, carry in," from in- "against" (see in- (1)) + vehere "to carry" (see vehicle). Meaning "to give vent to violent denunciation" is from 1520s. Related: Inveighed; inveighing.
inveigle (v.) Look up inveigle at Dictionary.com
late 15c., "to blind (someone's) judgment," 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.)). Loan-translation of Greek ap ommaton "without eyes." Meaning "to win over by deceit, seduce" is 1530s.
invent (v.) Look up invent at Dictionary.com
late 15c., "find, discover," a back-formation from invention or else from Latin inventus, past participle of invenire “to come upon; devise, discover” (see invention). Meaning "make up, think up" is from 1530s, as is that of "produce by original thought." Related: Invented; inventing.
invention (n.) Look up invention at Dictionary.com
c.1400, "devised method of organization," from Old French invencion (13c.) and directly from Latin inventionem (nominative inventio) "faculty of invention; a finding, discovery," noun of action from past participle stem of invenire "devise, discover, find," from in- "in, on" (see in- (2)) + venire "to come" (see venue).

Meaning "finding or discovering of something" is early 15c. in English; sense of "thing invented" is first recorded 1510s. Etymological sense 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.
inventive (adj.) Look up inventive at Dictionary.com
early 15c., "skilled in invention," from Old French inventif (15c.), from Latin invent-, past participle stem of invenire (see invention). Related: Inventively; inventiveness.
inventor (n.) Look up inventor at Dictionary.com
c.1500, "a discoverer," from Latin inventor (fem. inventrix) "contriver, author, discoverer," agent noun from past participle stem of invenire (see invention). Meaning "one who contrives or produces a new thing or process" is from 1550s.
inventory (n.) Look up inventory at Dictionary.com
early 15c., from Old French inventoire "inventory, detailed list of goods, catalogue," from Medieval Latin inventorium (Late Latin inventarium) "list of what is found," from Latin inventus, past participle of invenire "to find" (see invention). The verb is first recorded c.1600, from the noun.
inverse (adj.) Look up inverse at Dictionary.com
mid-15c., from Latin inversus, past participle of invertere (see invert). Related: Inversely. As a noun, 1680s, from the adjective.
inversion (n.) Look up inversion at Dictionary.com
1550s, from Latin inversionem (nominative inversio) "an inversion," noun of action from past participle stem of invertere (see invert).
invert (v.) Look up invert at Dictionary.com
1530s, from Middle French invertir or directly from Latin invertere "turn upside down, turn about," from in- "in, on" (see in- (2)) + vertere "to turn" (see versus). Related: Inverted; inverting; invertedly.
invertebrate (n.) Look up invertebrate at Dictionary.com
1826, from Latin in- "not" (see in- (1)) + vertebra "joint" (see vertebra). Invertebrata as a biological classification was coined 1805 by French naturalist Georges Léopole Chrétien Frédéric Dagobert, Baron Cuvier (1769-1832). As an adjective by 1838.