triad (n.)
1540s, "group or set of three," from Late Latin trias (genitive triadis), from Greek trias (genitive triados) "a triad, the number three," from treis "three" (see three). Musical sense of "chord of three notes" is from 1801. Related: Triadic.
triage (n.)
1727, "action of assorting according to quality," from French triage "a picking out, sorting" (14c.), from Old French trier "to pick, cull" (see try (v.)). There seems to be some influence from or convergence with Latin tria "three" (as in triage for "coffee beans of the third or lowest quality"). In World War I, adopted for the sorting of wounded soldiers into groups according to the severity of their injuries, from French use.
First of all, the wounded man, or "blessé, is carried into the first of the so-called "Salles de Triage" or sorting wards. Here his name and regimental number, and if he is in condition to give it, the address of his family are taken; .... Then a hasty look-over from the surgeon sends him into one of the two other "Salles de Triage" -- that of the "Petits Blessés" if he is only slightly wounded and that of the "Grands Blessés" if he is more severely so. [Woods Hutchinson, M.D., "The Doctor in War," Boston, 1918]
trial (n.)
mid-15c., "act or process of testing, a putting to proof by examination, experiment, etc.," from Anglo-French trial, noun formed from triet "to try" (see try (v.)). Sense of "examining and deciding of the issues between parties 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 (1826) translates French ballon d'essai, a small balloon sent up immediately before a manned ascent to determine the direction and tendency of winds in the upper air, though the earliest use in English is figurative.
triangle (n.)
late 14c., from Old French triangle (13c.), from Latin triangulum "triangle," noun use of neuter of adjective triangulus "three-cornered, having three angles," from tri- "three" (see tri-) + angulus "corner, angle" (see angle (n.)).
triangular (adj.)
c.1400, from Late Latin triangularis "triangular, pertaining to a triangle," from Latin triangulus "with three corners" (the usual adjective in classical Latin), as a noun, "a triangle;" see triangle. Related: Triangularly.
In the huts of witches all the instruments and implements are triangular. ["Handwörterbuch des deutschen Aberglaubens"]
triangularity (n.)
1680s, from triangular + -ity.
triangulate (v.)
1833, originally in surveying, from Latin triangulum "a triangle" (see triangle) + -ate (2). Related: Triangulated; triangulating. Figurative use by 1860.
triangulation (n.)
1809, from French triangulation, from Medieval Latin triangulationem (mid-12c., nominative triangulatio), noun of action from Latin *triangulare, from triangulum (see triangle).
triannual (adj.)
1630s, from tri- + annual (adj.). Related: Triannually.
Triassic (adj.)
1841, from German, coined 1841 by German geologist Friedrich August von Alberti (1795-1878), from Trias "period preceding the Jurassic," from Greek trias "triad, the number three" (see triad). So called because it is divisible (in Germany) into three groups.
triathlete (n.)
by 1983; see triathlon + athlete.
triathlon (n.)
1970, from tri- "three" + Greek athlon "contest;" formed on model of decathlon, biathlon, etc. Originally of various combinations of events; one of the earliest so called combined clay-pigeon shooting, fly-fishing, and horse-jumping; another was cross-country skiing, target shooting, and a giant slalom run; and a third connected to the U.S. Army involved shooting, swimming, and running. Applied to the combination of a long swim, a bicycle-race, and a marathon by 1981.
triaxial (adj.)
1886, from tri- + axial.
tribadism (n.)
"lesbian sexual activity," 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, rub down, wear away," from PIE root *tere- (1) "to rub, turn, twist" (see throw (v.)). In reference to a specific sexual technique, from 1965.
tribal (adj.)
1630s, "pertaining to or characteristic of tribes," from tribe + -al (1). 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. Related: Tribally.
tribalism (n.)
1868, "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 or directly 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.), of unknown origin. Perhaps from tri- "three" + *bheue-, root of the verb be. Others connect the word with the PIE root *treb- "a dwelling" (see tavern).

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 phylo-). Extension to modern ethnic groups or races of people is from 1590s, specifically "a division of a barbarous race of people, usually distinguishable in some way from their congeners, united into a community under a recognized head or chief" [Century Dictionary], but colloquially of any aggregate of individuals of a kind.
TriBeCa
1983, area in Manhattan between Broadway and the Hudson, south of Greenwich Village, from "triangle below Canal (Street)."
tribesman (n.)
1798, from genitive of tribe (n.) + man (n.).
tribology (n.)
1965, "study of friction," from comb. form of Greek tribos "rubbing," from tribein "to rub, rub down, wear away" (from PIE root *tere- (1) "to rub, turn, twist;" see throw (v.)) + -logy.
tribulation (n.)
c.1200, from Old French tribulacion (12c.), from Church Latin tribulationem (nominative tribulatio) "distress, trouble, affliction," noun of action from past participle stem 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 (v.)) + -bulum, suffix forming names of tools.
tribunal (n.)
early 15c., "a judgement seat," from Old French tribunal "justice seat, judgement seat" (13c.) and directly from Latin tribunal "platform for the seat of magistrates, elevation, embankment," from tribunus "official in ancient Rome, magistrate," literally "head of a tribe" (see tribune). 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" (in the Roman sense), from tribus (see tribe). Also "raised platform" (1762), from Italian tribuna, from Medieval Latin tribuna, from Latin tribunal in its classical sense "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).
tributary (n.)
late 14c., "person, country, etc. owing obedience or paying tribute or a tax to a sovereign or another people," from Latin tributarius (see tributary (adj.)). Meaning "stream that flows into a larger body" is from 1822, from the adjective in this sense, which is recorded from 1610s.
tribute (n.)
mid-14c., "stated sum of money or other valuable consideration paid by one ruler or country to another in acknowledgment of submission or as the price of peace or protection," from Anglo-French tribute, Old French tribut and directly from Latin tributum "tribute, a stated payment, a thing contributed or paid," noun use of neuter of tributus, 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 (v.)
late 14c., "haul up and fasten with a rope," 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 is 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" (see tri-) + -ceps, from caput "head" (see capitulum). So called because the muscle has three origins.
triceratops (n.)
dinosaur genus, 1890, 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," 1864, coined by Bernhard Rupprecht (1815-1877) by 1864 from trichina (1835), Modern Latin, genus name of the minute, hair-like parasitic worms that cause it, from Greek trikhine, fem. of trikhinos "of or like hair," from thrix (genitive trikhos) "hair."
trichomoniasis (n.)
1915, with -iasis + trichomonas, genus of a family of flagellate parasites, from tricho-, Latinized form of Greek trikho-, comb. form of thrix (genitive trikhos) "hair" + -monas.
trichotillomania (n.)
1905, from French trichotillomanie (1889), from tricho-, Latinized form of Greek trikho-, comb. form of thrix (genitive trikhos) "hair" + Greek tillesthai "to pull out" + mania.
trick (n.)
early 15c., "a cheat, a mean ruse," from Old North French trique "trick, deceit, treachery, cheating," from trikier "to deceive, to cheat," variant of Old French trichier "to cheat, trick, deceive," of uncertain origin, probably from Vulgar Latin *triccare, from Latin 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. Meaning "prostitute's client" is first attested 1915; earlier it was U.S. slang for "a robbery" (1865). To do the trick "accomplish one's purpose" is from 1812; to miss a trick "fail to take advantage of opportunity" is from 1889; from 1872 in reference to playing the card-game of whist, which might be the original literal sense. Trick-or-treat is recorded from 1942. Trick question is from 1907.
trick (v.)
"deceive by trickery," 1590s, from trick (n.). Related: Tricked; tricking. The sense of "to dress, adorn" (c.1500) is perhaps a different word entirely.
trickery (n.)
1719, from trick (v.) + -ery.
trickle (n.)
1570s, from trickle (v.).
trickle (v.)
late 14c., intransitive, of uncertain origin, possibly a shortened variant of stricklen "to trickle," a frequentative form of striken "to flow, move" (see strike (v.)). Transitive sense from c.1600. Related: Trickled; trickling. Trickle-down as an adjectival phrase in an economic sense first recorded 1944; the image had been in use at least since Teddy Roosevelt.
trickster (n.)
1711, from trick (n.) + -ster.
tricky (adj.)
1786, "characterized by tricks," from trick (n.) + -y (2). Meaning "deceptively difficult" is from 1868. Related: Trickily; trickiness. Earlier was tricksy (1590s).
tricolor (n.)
also tricolour, 1798, "flag having three colors," especially the emblem of France adopted during the Revolution, from French tricolore, in drapeau tricolore "three-colored flag." The arrangement of colors on the modern French national flag dates from 1794.
tricot (n.)
knitted fabric, 1859, from French tricot "knitting, knitted work," from tricoter "to knit," of uncertain origin, probably a variant of Old French estriquer "to smooth," from a Germanic source (such as Middle Low German striken "pass over lightly").
tricuspid (n.)
1660s, from Latin tricuspidem (nominative tricuspis) "three-pointed," from tri- (see tri-) + cuspis "point" (see cusp).
tricycle (n.)
1828, "three-wheeled horse-drawn carriage," from French tricycle (1827); see tri- + cycle (n.). The pedal-powered version is first attested 1868.
trident (n.)
"three-pointed spear," mid-15c., from Latin noun use of adjective tridentem (nominative tridens) "three-pronged, three-toothed," from tri- "three" (see tri-) + dens "tooth" (see tooth). As a type of U.S. nuclear-powered submarine, recorded from 1972. Related: Tridental.
tried (adj.)
"tested, proven, trusty," mid-14c., past participle adjective from try (v.). Coupled since mid-14c. with true.
triennial (adj.)
1630s, "lasting three years;" 1640s, "occurring every three years," with -al (1) + Latin triennium "three-year period," from tri- "three" (see tri-) + annus "year" (see annual). For vowel change, see biennial. As a noun, 1630s. Related: Triennially.
Trier
city in Germany (French Trèves), founded c.15 B.C.E. by Augustus, named for the indigenous Gaulish people, the Treveri.
trifecta (n.)
1974, from tri- + perfecta.
trifid (adj.)
"divided into three lobes," 1620s, from Latin trifidus "cleft in three," from tri- "three" (see tri-) + -fid. This adjective probably inspired triffid, the name of the three-legged walking poisonous plants in John Wyndham's novel "The Day of the Triffids" (1951).
trifle (n.)
c.1200, trufle "false or idle tale," later "matter of little importance" (c.1300), from Old French trufle "mockery," diminutive of truffe "deception," of uncertain origin. As a type of light confection from 1755.