Online Etymology Dictionary

tremulous (adj.): 1610s, from Latin tremulus "shaking, quivering," from tremere (see tremble).
trench (n.): late 14c., "track cut through a wood," later "long, narrow ditch" (late 15c.), from Old French trenche "a slice, ditch" (late 13c.), from trenchier "to cut," possibly from Vulgar Latin *trincare, from Latin truncare "to cut or lop off" (see truncate). Trenches for military protection are first so called c.1500. Trench warfare first attested 1918. Trench-coat first recorded 1916, a type of coat worn by British officers in the trenches.
trenchant (adj.): early 14c., "cutting, sharp," from Old French trenchant "cutting, sharp," present participle of trenchier "to cut" (see trench). Figurative sense is recorded from c.1600.
trencher (n.): c.1300, "wooden platter on which to cut meat," from Anglo-French trenchour, from Old North French trencheor "a trencher," literally "a cutting place," from Old French trenchier "to cut" (see trench).
trend (v.): 1590s, "to run or bend in a certain direction" (of rivers, coasts, etc.), from Middle English trenden "to roll about, turn, revolve," from Old English trendan, from Proto-Germanic *trandijanan (cf. Old English trinde "round lump, ball," Old Frisian trind, Middle Low German trint "round," Middle Low German trent "ring, boundary," Dutch trent "circumference," Danish trind "round"); origin and connections outside Germanic uncertain. Sense of "have a general tendency" (used of events, opinions, etc.) is first recorded 1863, from the nautical sense. Related: Trended; trending.
trend (n.): "the way something bends" (coastline, mountain range, etc.), 1777, from trend (v.); sense of "general tendency" is from 1884.
trendsetter (n.): also trend-setter, 1960; see trend (n.) + set (v.). Related: Trend-setting.
trendy (adj.): 1962, from trend (n.) + -y (2). Related: Trendily; trendiness.
Trent: river in England, from Celtic, perhaps “great wanderer,” in reference to its flooding. The city in Italy (Italian Trento) is Roman Tridentum, in reference to the triple-peaked mountain nearby. The great ecumenical council there was from 1543-63.
Trenton: city in New Jersey, U.S., originally Trent’s Town, from William Trent, Philadelphia merchant who laid it out in 1714.
trepan (v.): c.1400, from French trépaner (14c.), from Medieval Latin trepanum "a saw for cutting out small pieces of bone from the skull," from Greek trypanon, from trypan "to bore," related to trype "hole" (cf. Old Church Slavonic truplu "hollow"), from PIE root *tere- "to rub, turn" (see throw). Related: Trepanned; trepanning.
trephination (n.): 1874, from trephine (v.), 1804, from trephine (n.), 1620s, said to be from Latin tres fines "three ends," with form apparently influenced by trepan. Related: Trephining.
trepid (adj.): 1640s, from Latin trepidus "scared" (see trepidation).
trepidation (n.): c.1600, from Latin trepidationem (nominative trepidatio) "agitation, alarm, trembling," noun of action from past participle stem of trepidare "to tremble, hurry," from trepidus "alarmed, scared," from PIE *trep- "to shake, tremble" (cf. Sanskrit trprah "hasty," Old Church Slavonic trepetati "to tremble"), related to *trem- (see tremble).
tres: "very," 1815, from French très, from Old French tres, from Latin trans "beyond" (see trans-), later "very" (cf. Old Italian trafreddo "very cold").
trespass (v.): c.1300, "transgress, offend, sin," from Old French trespasser "pass beyond or across," from tres- "beyond" (from Latin trans-) + passer "go by, pass" (see pass (v.)). Meaning "enter unlawfully" is first attested in forest laws of Scottish Parliament (c.1455). The noun is recorded from late 13c. The modern descendant of Old French trespasser, French trépasser has come to be used euphemistically for "to die" (cf. cross over, and obituary).
tress (n.): late 13c., "long lock of hair," from Old French tresse "a plait or braid of hair" (12c.), perhaps from Vulgar Latin *trichia "braid, rope," from Greek trikhia "rope," from thrix (genitive trikhos) "hair."
tressel (n.): see trestle.
trestle (n.): early 14c., "a support for something," from Old French trestel "crossbeam" (12c.), presumed to be an alteration of Latin *transtellum, diminutive of transtrum "beam, crossbar." Specific meaning "support for a bridge" is recorded from 1796.
tret: c.1500, of unknown origin.
Trevor: surname, from Trevor, Denbeigh.
trey (n.): late 14c., "card, die, or domino with three spots," from Old French treis, oblique case of treie "three," from Latin tria (neuter) "three" (see three).
tri-: word-forming element meaning "three," from Latin tres (neuter tria) or Greek treis (neuter tria) "three" (see three).
triad (n.): 1540s, "group or set of three," from Late Latin trias (genitive triadis), from Greek trias (genitive triados), from treis "three" (see three). Musical sense of "chord of three notes" is from 1801.
triage (n.): 1727, "action of assorting according to quality," from French triage "a picking out, sorting," from Old French trier "to pick, cull" (see try). There seems to be some influence from or convergence with Latin tria "three" (e.g. triage for "coffee beans of the third or lowest quality"). In World War I, adopted for the sorting of wounded soldiers into three groups according to the severity of their injuries.
trial (n.): mid-15c., "act or process of testing," from Anglo-French trial, noun formed from triet "to try" (see try). Sense of "examining and deciding a case in a court of law" is first recorded 1570s; extended to any ordeal by 1590s. As an adjectival phrase, trial-and-error is recorded from 1806. Trial balloon (1939) is congnate of French ballon d'essai.
triangle (n.): late 14c., from Old French triangle (13c.), from Latin triangulum "triangle," from neuter of adj. triangulus "three-cornered," from tri- "three" (see tri-) + angulus "corner, angle" (see angle (n.)).
In the huts of witches all the instruments and implements are triangular. ["Handwörterbuch des deutschen Aberglaubens"]
triangular (adj.): 1540s, from Late Latin triangularis, from triangulum (see triangle).
triangulate (v.): 1833, originally in surveying,” from Latin triangulum (see triangle) + -ate (2). Related: Triangulated; triangulating. Figurative use by 1860.
triangulation (n.): 1818, from Medieval Latin triangulationem (mid-12c., nominative triangulatio), noun of action from Latin *triangulare, from triangulum (see triangle).
triannual (adj.): 1630s, from tri- + annual. Related: Triannually.
Triassic (adj.): 1841, from German, coined 1841 by German geologist Friedrich August von Alberti (1795-1878), from Greek trias "triad," because it is divisible (in Germany) into three groups.
triathlete (n.): by 1983; see triathlon + athlete.
triathlon (n.): 1973, from Greek tri- "three" (see tri-) + athlon "contest;" formed on model of decathlon, etc.
tribadism (n.): 1811, with -ism + tribade (n.), c.1600, "a lesbian," from French tribade (16c.) or directly from Latin tribas, from Greek tribas, from tribein "to rub," from PIE root *ter- "to rub, turn, twist" (see throw). In reference to a specific sexual technique, from 1965.
tribal (adj.): 1630s, "pertaining to tribes," from tribe + -al (1). Related: Tribally Meaning "characterized by a strong sense of loyalty to one's group" is from 1951 (Arendt). As a style of belly-dance from 1999, American English.
tribalism (n.): 1886, "condition of being a tribe," from tribal + -ism. Meaning "group loyalty" attested by 1955.
tribe (n.): mid-13c., "one of the twelve divisions of the ancient Hebrews," from Old French tribu, from Latin tribus "one of the three political/ethnic divisions of the original Roman state" (Tites, Ramnes, and Luceres, corresponding, perhaps, to the Latins, Sabines, and Etruscans), later, one of the 30 political divisions instituted by Servius Tullius (increased to 35 in 241 B.C.E.), perhaps from tri- "three" + *bhu-, root of the verb be. Others connect the word with the root of Welsh tref "town, inhabited place."

In the Biblical sense, which was the original one in English, the Latin word translates Greek phyle "race or tribe of men, body of men united by ties of blood and descent, a clan" (see physic). Extension to any ethnic group or race of people is first recorded 1590s.
TriBeCa: 1983, area in Manhattan between Broadway and the Hudson, south of Greenwich Village, from "triangle below Canal (Street)."
tribology (n.): 1965, "study of friction," from Greek tribos "rubbing," from tribein "to rub" (from PIE root *ter- "to rub, turn, twist;" see throw) + -logy.
tribulation (n.): early 13c., from Old French tribulacion (12c.), from Late Latin tribulationem (nominative tribulatio) "distress, trouble, affliction" (c.200), from tribulatus, past participle of tribulare "to oppress, afflict," a figurative use by Christian writers of Latin tribulare "to press," also possibly "to thresh out grain," from tribulum "threshing sledge," from stem of terere "to rub" (see throw) + -bulum, suffix forming names of tools.
tribunal (n.): mid-15c., from Old French tribunal (13c.), from Latin tribunal "platform for the seat of magistrates, elevation, embankment," from tribunus "official in ancient Rome, magistrate," literally "head of a tribe," from tribus (see tribe). Hence, a court of justice or judicial assembly (1580s).
tribune (n.): late 14c., title of an official in ancient Rome, from Latin tribunus "magistrate" (specifically one of the officers appointed to protect the rights and interests of the plebeians from the patricians), originally "head of a tribe," from tribus (see tribe). The meaning "raised platform" is 1762, from Italian, from Latin tribunal "platform for the seats of magistrates in ancient Rome."
tributary (adj.): late 14c., "paying tribute," from Latin tributarius "liable to tax or tribute," from tributum (see tribute). The noun sense of "one who pays tribute" is recorded from early 15c. Meaning "stream that flows into a larger body" is from 1822, from the adjective, which is recorded from 1610s in this sense.
tribute (n.): mid-14c., "tax paid to a ruler or master for security or protection," from Latin tributum "tribute," literally "a thing contributed or paid," noun use of tributus, neuter past participle of tribuere "to pay, assign, grant," also "allot among the tribes or to a tribe," from tribus (see tribe). Sense of "offering, gift, token" is first recorded 1580s.
trice: late 14c., "haul up and fasten with a rope" (v.), from Middle Dutch trisen "hoist," from trise "pulley," of unknown origin. Hence at a tryse (mid-15c.) "in a very short time," literally "at a single pluck or pull." The Middle Dutch word is the source of Dutch trijsen "to hoist," and cognate with Middle Low German trissen (source of Danish trisse, German triezen); its ultimate origin is unknown.
triceps (n.): "the great extensor muscle," 1704, from Latin triceps "three-headed," from tri- "three" + -ceps, from caput "head" (see capitulum). So called because the muscle has three origins.
triceratops (n.): dinosaur genus, 1892, from Greek trikeratos "three-horned" + ops "face," literally "eye," from PIE *okw- "to see" (see eye (n.)). The first element is from tri- "three" (see three) + keras (genitive keratos) "horn" (see kerato-).
trichinosis (n.): "disease caused by trichinae," 1866, coined by Bernhard Rupprecht (1815-1877) from trichina (1835), Modern Latin, genus name of certain minute parasitic worms, from Greek trikhine, fem. of trikhinos "of or like hair," from thrix (genitive trikhos) "hair."
trichomoniasis (n.): 1915, from tricho-, Latinized form of Greek trikho-, comb. form of thrix (genitive trikhos) "hair" + -monas + -iasis. Coined 1836 in French.