Online Etymology Dictionary

intimidate (v.): 1640s, from Medieval Latin intimidatus, past participle of intimidare "to frighten, make afraid," from in- "in" (see in- (2)) + Latin timidus "fearful" (see timid). Related: Intimidated; intimidating. The French verb was intimider (16c.).
intimidation (n.): 1650s, noun of action from intimidate; perhaps modeled on French intimidation (16c.).
intire (adj.): obsolete form of entire. Related: Intirely.
into (prep.): Old English into "into, to, against, in," originally in to. It emerged in late Old English to do the work of the dative case inflections, then fading, that formerly distinguished, for instance, the notion of "in the house" from that of "into the house." Compare onto, unto. To be into (something) "be intensely involved in or devoted to" recorded by 1967 in American English youth slang.
intolerability (n.): 1590s, from intolerable + -ity or else from Late Latin intolerabilitas, from Latin intolerabilis "that cannot bear; that cannot be borne." Slightly earlier in the same sense was intolerableness.
intolerable (adj.): late 14c., from Latin intolerabilis "that cannot bear; that cannot be borne," from in- "not, opposite of" (see in- (1)) + tolerabilis "that may be endured," from tolerare "to bear, endure" (see toleration). Related: Intolerably.
intolerance (n.): 1765, "unwillingness to endure a differing opinion or belief," from Latin intolerantia "impatience; unendurableness, insufferableness; insolence," from intolerantem "impatient, intolerant" (see intolerant). There is an isolated use from c. 1500, with an apparent sense of "unwillingness." Especially of religious matters through mid-19c. Now-obsolete intolerancy was used in same sense from 1620s; intoleration from 1610s. Meaning "incapacity to bear or endure" is by 1844.
intolerant (adj.): 1735, "unable or unwilling to endure" (a condition, etc.), from Latin intolerantem (nominative intolerans) "not enduring, impatient, intolerant; intolerable," from in- "not" (see in- (1)) + tolerans, present participle of tolerare "to bear, endure" (see toleration).

Meaning "not disposed to endure contrary opinions or beliefs, impatient of dissent or opposition" is from 1765. Of plants, with reference to deep shade, from 1898. The noun meaning "person or persons who do not favor toleration" is from 1765. Related: Intolerantly.
intonate (v.1): "to intone, recite in a singing voice," 1795, from Medieval Latin intonatus, past participle of intonare "sing according to tone" (see intone). Compare Italian intonare, French entonner. Related: Intonated; intonating.
intonate (v.2): "to thunder, rumble," 1620s, from past participle stem of Latin intonare "to thunder, thunder forth," from in- (see in (2)) + tonare "to thunder" (see thunder (n.)). Related: Intonated; intonating.
intonation (n.): 1610s, "opening phrase of a melody," from French intonation (14c.), from Medieval Latin intonationem (nominative intonatio), noun of state from past participle stem of intonare (see intone). From 1788 as "action of intoning." Meaning "modulation of the voice in speaking, utterance of tones" is from 1791.
intone (v.1): late 14c., entunen "sing, chant, recite, vocalize," from Old French entoner "to sing, chant" (13c.), from Medieval Latin intonare "sing according to tone," from Latin in- "in" (see in- (2)) + tonus "tone," from Greek tonos (see tenet). Related: Intoned; intoning.
intone (v.2): obsolete 17c.-18c. verb, from French entoner "thunder, roar, resound, reverberate," from Latin intonare "to thunder, resound," figuratively "to cry out vehemently," from tonare "to thunder" (see thunder (n.)). Related: Intoned; intoning.
intoxicant (n.): "that which intoxicates," 1798; see intoxicate. Perhaps from Medieval Latin intoxicantem (nominative intoxicans), present participle of intoxicare. As an adjective from 1882.
intoxicate (v.): mid-15c., "to poison" (obsolete), from Medieval Latin intoxicatus, past participle of intoxicare "to poison," from in- "in" (see in- (2)) + Latin toxicare "to poison," from toxicum "poison" (see toxic). Meaning "make drunk" first recorded 1570s (implied in intoxicated). Figurative sense "excite to a high pitch of feeling" is attested from 1590s. Related: Intoxicating.
intoxicated (adj.): 1550s, "poisoned;" 1570s, "drunk," past participle adjective from intoxicate (v.).
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.
intra-: word-forming element meaning "within, inside, on the inside," from Latin preposition intra "on the inside, within, in, into;" of time, "during, in the course of," related to inter "between," from PIE *en-t(e)ro-, from root *en "in" (see in). Commonly opposed to extra-, and compare inter-. The use of intra as a prefix was rare in classical Latin.
intra-cellular (adj.): also intracellular, "existing or happening inside a cell," 1842; see intra- "within" + cellular.
intra-genic (adj.): also intragenic, "occurring within a gene," 1937, from intra- "within" + gene + -ic.
intra-ocular (adj.): also intraocular, 1826, from intra- + ocular.
intra-orbital (adj.): also intraorbital, 1836, from intra- "within" + orbit (n.) + -al (1).
intra-ovarian (adj.): 1845; see intra- + ovarian.
intra-peritoneal (adj.): also intraperitoneal, "within the cavity of the peritoeum," 1835, from intra- "within" + peritoneal.
intra-personal (adj.): also intrapersonal, 1853, from intra- "within" + personal.
intra-psychic (adj.): also intrapsychic, 1902, from intra- "within" + psychic.
intra-uterine (adj.): also intrauterine, 1820; see intra- "within" + uterine.
intractability (n.): 1570s, from intractable + -ity. Intractableness is from 1660s.
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.
intramercurial (adj.): "being within the orbit of the planet Mercury," 1859, especially in reference to a supposed planet orbiting there (sought in vain in the eclipse of 1860), from intra- "within, inside" + Mercury (Latin Mercurius) + -al (1). The idea originated in France in the 1840s with Urbain Le Verrier, who later became director of the Paris Observatory. There was some excitement about it in 1859 when a French doctor named Lescarbault claimed to have tracked it crossing the Sun's disk and convinced Le Verrier. It was sought in vain in the solar eclipses of 1860, '68, and '69. See Vulcan.
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).
intramuscular (adj.): also intra-muscular, 1874, from intra- "within" + muscle (Latin musculus) + -ar.
intrans.: abbreviation of intransitive (adj.).
intransigence (n.): 1874, from intransigent or else from or based on French intransigeance, from intransigent. Related: Intransigency (1883).
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.
intranslatable (adj.): 1680s, from in- (1) "not, opposite of" + translate + -able. Related: Itranslatably; intranslatability.
intranslatable (adj.): 1680s, from in- (1) "not, opposite of" + translate + -able. Related: Itranslatably; intranslatability.
intransmissible (adj.): 1650s, from in- (1) "not, opposite of" + transmissible. Related: Intransmissibly; intransmissibility.
intransmutable (adj.): 1690s, from in- (1) "not, opposite of" + transmute (v.) + -able). Related: Intransmutably; intransmutability.
intraspecific (adj.): 1905, from intra- + specific, here representing species (n.).
intravenous (adj.): "in or occurring within a vein," 1847, from intra- "within, inside" + Latin venous, from vena "vein" (see vein). Related: Intravenously.
intrench (v.): obsolete form of entrench (q.v.). Related: Intrenched; intrenchment.
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).
intrepidity (n.): 1640s, from intrepid (adj.) + -ity.
intricacy (n.): c. 1600, "state of being complex;" 1610s, "an intricate situation or condition," from intricate (adj.) + -cy. Related: Intricacies.
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.
intrigant (n.): also intriguant, "man given to intrigue," 1781, from French intrigant "male intriguer," from Italian intrigante, noun use of present participle of intrigare "to plot, meddle" (see intrigue (v.)).
intrigante (n.): also intriguante, "woman given to intrigue," 1806, from fem. of French intrigant "male intriguer," from Italian intrigante, noun use of present participle of intrigare "to plot, meddle" (see intrigue (v.)).
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.