Online Etymology Dictionary

Tripoli: both the Libyan capital and the Lebanese port city represent Greek tri- "three" + polis "town." In Libya, Tripolis was the name of a Phoenician colony consisting of Oea (which grew into modern Tripoli), Leptis Magna, and Sabratha. Arabic distinguishes them as Tarabulus ash-sham ("Syrian Tripoli") and Tarabulus al-garb ("Western Tripoli").
triptych (n.): 1731, "hinged, three-leaved writing tablet used in ancient Greece and Rome," from Greek triptykhos "three-layered," from tri- "three" + ptykhos, genitive of ptyx "fold, layer." In reference to three-part altar-piece carvings or pictures hinged together, it is attested from 1849.
trireme (n.): c.1600, "ancient ship with three rows of oars," from Latin triremis, from tri- "three" + remus "oar."
Triscuit (n.): proprietary name for a type of cracker, 1906, curiously from tri- + biscuit.
trisect (v.): 1660s (implied in trisection), from tri- "three" + Latin sectus "cut," past participle of secare "to cut" (see section). Probably patterned on bisect.
trisexual (adj.): by 1986, from tri- + sexual.
triskaidekaphobia (n.): "fear of the number 13," 1911, from Greek treiskaideka "thirteen" + -phobia "fear."
triskelion (n.): "figure consisting of three branchess radiating from a center," 1880, earlier triskelos (1857), from Greek triskeles "three-legged," from tri- "three" + skelos "leg" (see scalene).
trismus (n.): 1690s, Modern Latin, from Greek trismos "a scream; a grinding, rasping," akin to trizein "to chirp, gnash."
trisomy (n.): 1930, from trisomic (1921), from tri- + ending from chromosome.
trist (adj.): "sorrowful," early 15c., from French triste (10c.), from Latin tristis "sad, sorrowful, gloomy. " Formerly nativized, since late 18c. treated as a French word in English and often spelled triste.
Tristram: masc. proper name, name of a medieval hero, from Welsh Drystan, influenced by French triste "sad."
trite (adj.): 1540s, from Latin tritus "worn, familiar," from past participle of terere "to rub, wear down" (see throw).
triticale (n.): hybrid cereal grass, 1952, from Modern Latin Triti(cum) "wheat" (literally "grain for threshing," from tritus, past participle of terrere "to rub, thresh, grind") + (Se)cale "rye."
tritium (n.): 1933, Modern Latin, from Greek tritos "third" (see third) + chemical suffix -ium.
Triton: minor sea god, son of Poseidon and Amphitrite, from Greek Triton, cognate with Old Irish triath (genitive trethan) "sea."
triturate (v.): 1755, from Late Latin trituratus, past participle of triturare "to thresh, to grind," from Latin tritura "a rubbing, a threshing," from past participle stem of terere "to rub" (see throw). Related: Triturated; triturating.
triumph (n.): late 14c., from Old French triumphe (12c.), from Latin triumphus "achievement, a success, procession for a victorious general or admiral," earlier triumpus, probably via Etruscan from Greek thriambos "hymn to Dionysus," a loan-word from a pre-Hellenic language. Sense of "victory, conquest" is c.1400.
triumph (v.): late 15c.; see triumph (n.). Related: Triumphed; triumphing.
triumphant (adj.): late 15c., from Latin triumphantem, present participle of triumphare (see triumph).
triumphantly (adv.): 1540s, from triumphant + -ly (2).
triumvir (n.): "one of three men in the same office or of the same authority," 1570s, from Latin triumvir, from Old Latin phrase trium virum, genitive plural of tres viri "three men," from tres "three" + viri, plural of vir "man" (see virile).
triumvirate (n.): 1580s, from Latin triumviratus, from triumvir (see triumvir).
triune: "three in one," 1630s, from tri- + Latin unus "one" (see one).
trivet (n.): three-legged iron stand, 12c., trefet, probably from Latin tripedem (nominative tripes) "three-footed," from tri- "three" (see three) + pes "foot" (see foot (n.)).
trivia (n.): "trivialities, things of little consequence," 1902, popularized as title of a book by L.P. Smith, from L. trivia, plural of trivium "place where three roads meet" (see trivial).
trivial (adj.): early 15c., "of the trivium," from Medieval Latin trivialis, from trivium "first three of the seven liberal arts," from Latin, literally "place where three roads meet," from tri- "three" (see three) + via "road" (see via). The basic notion is of "that which may be found anywhere, commonplace, vulgar." The meaning "ordinary" (1580s) and "insignificant" (1590s) were in Latin trivialis "commonplace, vulgar," originally "of or belonging to the crossroads."
triviality (n.): 1590s, "quality of being trivial," from trivial + -ity. Meaning "a trivial thing or affair" is from 1610s. Related: Trivialities.
trivialize (v.): 1846, from trivial + -ize. Related: Trivialized; trivializing.
trivium (n.): 1804, from Medieval Latin, "grammar, rhetoric, and logic," first three of the seven liberal arts in the Middle Ages, considered less important than arithmetic, geometry, astronomy, and music. From Latin trivium "place where three roads meet" (see trivial).
trochaic (adj.): 1580s, from French trochaïque (1540s) or directly from Latin trochaicus, from Greek trokhaikos, from trokhaios (see trochee).
trochanter: 1610s, from French trochanter (16c.), from Greek trokhanter, from trekhein "to run" (see truckle (n.)).
trochee (n.): 1580s, from French trochée, from Latin trochaeus "a trochee," from Greek trokhaios (pous), literally "a running, spinning (foot)," from trekhein "to run" (see truckle (n.)). As a metrical foot, a long followed by a short syllable, or an accented followed by an unaccented one.
trod: past tense of tread (q.v.).
trodden (adj.): "that has been stepped on," 1540s, from past participle of tread (v.). The past participle was altered from Middle English treden under influence of Middle English past participles such as stolen from steal.
trog (n.): "obnoxious person, boor, lout," 1956, short for troglodyte.
troglodyte (n.): "cave-dweller," 1550s, from Latin troglodytae (plural), from Greek troglodytes "cave-dweller," literally "one who creeps into holes," from trogle "hole" (from trogein "to gnaw;" see trout) + dyein "go in, dive in."
troika (n.): 1842, "carriage drawn by three horses abreast," from Russian troika "three-horse team, any group of three," from collective numeral troe "three" + diminutive suffix -ka. Sense of "any group of three administrators, triumvirate" is first recorded 1945.
Trojan (adj.): Old English Troian "of or pertaining to ancient Troy," from Latin Trojanus, from Troia, Troja "Troy," from Greek Tros, name of a king of Phrygia, the mythical founder of Troy. In 17c., it was a colloquial term for "person of dissolute life, carousing companion." The trade name for a brand of prophylactic contraceptive was registered 1927 in U.S. Trojan horse is attested from 1570s; the computer virus sense is attested by 1982.
troll (v.): late 14c., "to go about, stroll," later (early 15c.) "roll from side to side, trundle," from Old French troller, a hunting term, "wander, to go in quest of game without purpose," from a Germanic source (cf. Old High German trollen "to walk with short steps"), from Proto-Germanic *truzlanan.

Sense of "sing in a full, rolling voice" (first attested 1570s) and that of "fish with a moving line" (c.1600) are both extended technical applications of the general sense of "roll, trundle," the latter perhaps confused with trail or trawl. Figurative sense of "to draw on as with a moving bait, entice, allure" is from 1560s. Meaning "to cruise in search of sexual encounters" is recorded from 1967, originally in homosexual slang.
troll (n.): "ugly dwarf or giant," 1610s, from Old Norse troll "giant, fiend, demon." Some speculate that it originally meant "creature that walks clumsily," and derives from Proto-Germanic *truzlan, from *truzlanan (see troll (v.)). But it seems to have been a general supernatural word, cf. Swedish trolla "to charm, bewitch;" Old Norse trolldomr "witchcraft."

The old sagas tell of the troll-bull, a supernatural being in the form of a bull, as well as boar-trolls. There were troll-maidens, troll-wives, and troll-women; the trollman, a magician or wizard, and the troll-drum, used in Lappish magic rites. The word was popularized in English by 19c. antiquarians, but it has been current in the Shetlands and Orkneys since Viking times. The first record of it is from a court document from the Shetlands, regarding a certain Catherine, who, among other things, was accused of "airt and pairt of witchcraft and sorcerie, in hanting and seeing the Trollis ryse out of the kyrk yeard of Hildiswick."

Originally conceived as a race of giants, they have suffered the same fate as the Celtic Danann and are now regarded in Denmark and Sweden as dwarfs and imps supposed to live in caves or under the ground.
trolley (n.): 1823, in Suffolk dialect, "a cart," especially one with wheels flanged for running on a track (1858), probably from troll (v.) in the sense of "to roll." Sense transferred to "pulley to convey current to a streetcar motor" (1890), then "streetcar drawing power by a trolley" (1891).
trollop: 1610s, "slovenly woman," probably from troll (v.) in sense of "roll about, wallow."
[A] certain Anne Hayward, wife of Gregory Hayward of Beighton, did in the parishe church of Beighton aforesaid in the time of Divine Service or Sermon there, and when the Minister was reading & praying, violently & boisterously presse & enter into the seat or place where one Elizabeth, wife of Robert Spurlinir, was quietly at her Devotion & Duty to Almighty God and then and there did quarrel chide & braule & being evilly & inalitiously bent did use then and there many rayleing opprobrious Speeches & Invectives against the said Elizabeth calling her Tripe & Trallop, to the great disturbance both of the Minister and Congregation. [Archdeaconry of Sudbury, Suffolk, Court Proceedings, 1682]
trombone (n.): brass wind instrument, 1724, from Italian trombone, augmentative form of tromba "trumpet," from a Germanic source (cf. Old High German trumba "trumpet;" see trumpet).
tromp (v.): 1892, variant of tramp; mainly American English. Related: Tromped; tromping.
trompe l'oeil: 1889, French, literally "deceives the eye."
troop (n.): 1540s, "body of soldiers," from Middle French troupe, from Old French trope "band of people, company, troop" (13c.), probably from Frankish *throp "assembly, gathering of people" (cf. Old English ðorp, Old Norse thorp "village," see thorp). OED derives the French word from Latin troppus "flock," which is of unknown origin but may be from the Germanic source.
troop (v.): 1560s, "to assemble," from troop (n.). Meaning "to march" is recorded from 1590s; that of "to go in great numbers, to flock" is from c.1600. Related: Trooped; trooping.
trooper (n.): 1630s, "soldier in a cavalry troop," agent noun from troop. Extended to "mounted policeman" (1858, in Australian) then to "state policeman" (U.S.) by 1911.
trope (n.): 1530s, from Latin tropus "a figure of speech," from Greek tropos "turn, direction, turn or figure of speech," related to trope "a turning" and trepein "to turn," from PIE root trep- "to turn" (cf. Sanskrit trapate "is ashamed, confused," properly "turns away in shame;" Latin trepit "he turns"). Technically, in rhetoric, a figure of speech which consists in the use of a word or phrase in a sense other than that which is proper to it.