Online Etymology Dictionary

trey: late 14c., "card, die, or domino with three spots," from O.Fr. treis, oblique case of treie "three," from L. tria (neut.) "three" (see three).
tri-: comb. form meaning "three," from L. tres (neut. tria) or Gk. treis (neut. tria) "three" (see three).
triad: 1546, "group or set of three," from L.L. trias (gen. triadis), from Gk. trias (gen. triados), from treis "three" (see three). Musical sense of "chord of three notes" is from 1801.
triage: 1727, "action of assorting according to quality," from Fr. triage "a picking out, sorting," from O.Fr. trier "to pick, cull" (see try). There seems to be some influence from or convergence with L. 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: mid-15c., "act or process of testing," from Anglo-Fr. 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.
triangle: late 14c., from O.Fr. triangle (13c.), from L. triangulum "triangle," from neut. of adj. triangulus "three-cornered," from tri- "three" + angulus "corner, angle." Triangulation is recorded from 1818.
In the huts of witches all the instruments and implements are triangular. ["Handwörterbuch des deutschen Aberglaubens"]
Triassic: 1841, from Ger., coined 1841 by Ger. geologist Friedrich August von Alberti (1795-1878), from Gk. trias "triad," because it is divisible (in Germany) into three groups.
triathlon: 1973, from Gk. tri- "three" + athlon "contest;" formed on model of decathlon, etc.
tribe: mid-13c., "one of the twelve divisions of the ancient Hebrews," from O.Fr. tribu, from L. 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 Gk. 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. Tribal "characterized by strong group loyalty" is recorded from 1951.
TriBeCa: 1983, area in Manhattan between Broadway and the Hudson, south of Greenwich Village, from "triangle below Canal (Street)."
tribology: 1965, "study of friction," from Gk. tribos "rubbing," from tribein "to rub" (from PIE base *ter- "to rub, turn, twist;" see throw) + -logy.
tribulation: early 13c., from O.Fr. tribulacion (12c.), from L.L. tribulationem (nom. tribulatio) "distress, trouble, affliction" (c.200), from tribulatus, pp. of tribulare "to oppress, afflict," a figurative use by Christian writers of L. 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: mid-15c., from O.Fr. tribunal (13c.), from L. tribunal "platform for the seat of magistrates, elevation, embankment," from tribunus "official in ancient Rome, magistrate," lit. "head of a tribe," from tribus (see tribe). Hence, a court of justice or judicial assembly (1580s).
tribune: late 14c., "official in ancient Rome," from L. 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 It., from L. tribunal "platform for the seats of magistrates in ancient Rome."
tributary (adj.): late 14c., "paying tribute," from L. 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 adj., which is recorded from 1610s in this sense.
tribute: mid-14c., "tax paid to a ruler or master for security or protection," from L. tributum "tribute," lit. "a thing contributed or paid," noun use of tributus, neuter pp. 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 M.Du. trisen "hoist," from trise "pulley," of unknown origin. Hence at a tryse (mid-15c.) "in a very short time," lit. "at a single pluck or pull." The M.Du. word is the source of Du. trijsen "to hoist," and cognate with M.L.G. trissen (source of Dan. trisse, Ger. triezen); its ultimate origin is unknown.
triceps: "the great extensor muscle," 1704, from L. triceps "three-headed," from tri- "three" + -ceps, from caput "head" (see head). So called because the muscle has three origins.
triceratops: dinosaur genus, 1892, from Gk. trikeratos "three-horned" + ops "face." The first element is from tri- "three" + keras (gen. keratos) "horn."
trichinosis: "disease caused by trichinae," 1866, coined by Bernhard Rupprecht (1815-77) from trichina (1835), from Mod.L., genus name of certain minute parasitic worms, from Gk. trikhine, fem. of trikhinos "of or like hair," from thrix (gen. trikhos) "hair."
trick (n.): early 15c., "a cheat, a mean ruse," from O.N.Fr. trique "trick, deceit, treachery, cheating," from trikier "to deceive, to cheat," variant of O.Fr. trichier, probably from V.L. *triccare, from L. tricari "be evasive, shuffle," from tricæ "trifles, nonsense, a tangle of difficulties," of unknown origin. Meaning "a roguish prank" is recorded from 1580s; sense of "the art of doing something" is first attested 1610s. The verb is first attested 1590s. Related: Tricked; tricking. An earlier sense of "to dress, adorn" (c.1500) is perhaps a different word entirely. Meaning "prostitute's client" is first attested 1915; earlier it was U.S. slang for "a robbery" (1865). Trickery is first attested 1800; tricky is 1786 (earlier tricksy, 1590s); trickster is from 1711. Trick-or-treat is recorded from 1947.
trickle (v.): late 14c., possibly an aphetic variant of stricklen "to trickle," a frequentative form of striken "to flow, move" (see strike). The noun is 1580, from the verb. Trickle-down as an adjectival phrase in an economic sense first recorded 1944; the image had been in use at least since Teddy Roosevelt.
tricolor: 1798, "flag having three colors," especially the emblem of France adopted during the Revolution, from Fr. tricolore, in drapeau tricolore "three-colored flag." The arrangement of colors on the Fr. flag dates from 1794.
tricot: 1859, from Fr. tricot "knitting, knitted work," from tricoter "to knit," probably a variant of O.Fr. estriquer "to smooth," from a Gmc. source (e.g. M.L.G. striken "pass over lightly").
tricycle: 1828, "three-wheeled horse-drawn carriage," from Fr. tricycle (1827), from tri- + cycle. The pedal-powered version is first attested 1868. Short form trike (on the model of bike) is attested from 1883.
trident: mid-15c., from L. tridentem (nom. tridens; gen. tridentis) "three-pronged, three-toothed," from tri- "three" + dens (gen. dentis) "tooth" (see tooth). As a type of U.S. nuclear-powered submarine, recorded from 1972.
triennial: 1640, "lasting three years;" 1642 in the sense of "occurring every three years," from L. triennium "three-year period," from tri- "three" + annus "year" (see annual).
trifid: "divided into three lobes," 1620s, from L. trifidus "cleft in three," from tri- "three" + -fid. This adj. probably inspired triffid, the name of the three-legged walking poisonous plants in John Wyndham's novel "The Day of the Triffids" (1951).
trifle: early 13c., trufle "false or idle tale," later "matter of little importance" (late 13c.), from O.Fr. trufle "mockery," dim. of truffe "deception," of uncertain origin. The verb, in the sense of "treat lightly," is first attested 1520s.
trig: "smart, trim," c.1200, from O.N. tryggr "firm, trusty, true" (see true). A Scottish and northern word only until 19c.
trigger (n.): 1660, earlier tricker (1621), from Du. trekker "trigger," from trekken "to pull" (see trek). Tricker was the usual form in Eng. until c.1750. The verb is first recorded 1930, from the noun. Trigger-happy is attested from 1943.
trigonometry: "branch of mathematics that deals with relations between sides and angles of triangles," 1614, from Mod.L. trigonometria (Barthelemi Pitiscus, 1595), from Gk. trigonon "triangle" (from tri- "three" + gonia "angle;" see knee) + metron "a measure" (see meter (2)).
trilby: type of hat, 1897, in allusion to Trilby O'Ferrall, eponymous heroine of the novel by George du Maurier (1834-96), published in 1894. In the stage version of the novel, the character wore this type of soft felt hat. In plural, also slang for "feet" (1895), in allusion to the eroticism attached to the heroine's bare feet.
trill (n.): 1649, from It. trillio, triglio "a quavering or warbling in singing," probably of imitative origin. The verb is 1666, from It. trillare "to quaver, trill."
trillion: 1680s, from Fr. trillion (late 15c.), from tri- + (m)illion. Cf. billion. In the U.S., the fourth power of a thousand (one thousand billion, 1 followed by 12 zeroes); in Great Britain, the third power of a million (one million billion, 1 followed by 18 zeroes), which is the original sense.
trilobite: extinct marine arthropod, 1832, from Mod.L. Trilobites (Walch, 1771), from Gk. tri- "three" + lobos "lobe," so called because its body is divided into three lobes.
trilogy: 1661, "any series of three related works," from Gk. trilogia "series of three related tragedies performed at Athens at the festival of Dionysus," from tri- "three" + logos "story."
trim (v.): probably from O.E. trymman "strengthen, make ready," from trum "strong, stable," from P.Gmc. *trumaz; said to be cognate with Skt. drumah "tree," Gk. drymos "copse, thicket," drys "tree, oak," and O.E. treow (see tree). Examples in M.E. are wanting. Original sense is preserved in nautical phrase in fighting trim (see trim (n.)). Meaning "make neat by cutting" is first recorded 1520s; that of "decorate, adorn" is from 1540s. Sense of "reduce" is attested from 1966. The adj. sense of "in good condition, neat, fit" is attested from c.1500, probably ultimately from O.E. adj. trum.
trim (n.): "state of being prepared," 1590, nautical jargon, from trim (v.). The meaning "visible woodwork of a house" is recorded from 1884; sense of "ornamental additions to an automobile" is from 1922. Slang meaning "a woman regarded as a sex object" is attested from 1955, Amer.Eng.
trimester: 1821, "period of three months," from Fr. trimestre (early 17c.), from L. trimestris "of three months," from tri- "three" + mensis "month" (see moon (n.)). Specific obstetrics sense is attested from 1916.
trimeter: "a verse of three metrical feet," 1567, from L. trimetrus, from Gk. trimetros "having three measures," from tri- "three" + metron "a measure" (see meter (2)).
trimmer: "one who changes opinions, actions, etc. to suit circumstances," 1682, from trim (v.) in a nautical sense of "distribute the load of a ship so she floats on an even keel" (1580), hence, "to adjust the balance of sails or yards with reference to the wind's direction" (1624).
trimmings: "adornments, accessories, etc.," 1612, from trim (v.).
trinity: early 13c., "the Father, Son and Holy Spirit," constituting one God in prevailing Christian doctrine, from O.Fr. trinite (11c.), from L. trinitatem (nom. trinitas) "Trinity, triad" (Tertullian), from trinus "threefold, triple," from pl. of trini "three at a time, threefold," related to tres (neut. tria) "three." The L. word was widely borrowed in European languages with the rise of Christianity (e.g. Ir. trionnoid, Welsh trindod, Ger. trinität).
trinket: 1533, of unknown origin. Evidently a dim. form, perhaps related to trick.
trio: 1724, from Fr. trio (c.1600), from It. trio, from tri- "three;" patterned on duo.
trip (v.): late 14c. (implied in tripper), "tread or step lightly, skip, caper," from O.Fr. tripper "strike with the feet" (12c.), from a Germanic source (cf. M.Du. trippen "to skip, trip, hop," Low Ger. trippeln, Fris. tripje, Du. trappen, O.E. treppan "to tread, trample") related to trap. The sense of "strike with the foot and cause to stumble" is first recorded early 15c. Meaning "to release" (a catch, lever, etc.) is recorded from 1897; trip-wire is attested from 1916. Related: Tripped; tripping.
trip (n.): "act or action of tripping," 1660, from trip (v.); sense of "a short journey or voyage" is from 1691, originally a nautical term, the connection is uncertain. The meaning "psychedelic drug experience" is first recorded 1959 as a noun; the verb in this sense is from 1966, from the noun.
tripartite: early 15c., "divided in three," from L. tripartitus "divided into three parts," from tri- "three" + partitus, pp. of partiri "to divide" (see part (v.)).
tripe: c.1300, from O.Fr. tripe "entrails used as food" (13c.), of unknown origin, perhaps via Sp. tripa from Arabic therb "suet" (but also said to mean "fold of a piece of cloth"). Applied contemptuously to persons (1595), then to anything considered worthless, foolish, or offensive (1892).