Online Etymology Dictionary

thorax (n.): "chest," c.1400, from Latin thorax, from Greek thorax (genitive thorakos) "breastplate, chest," of unknown origin.
Thorazine (n.): central nervous system depressant, 1954, proprietary name (Smith, Kline & French) formed from a rearrangement of various elements in the full chemical name.
thorium (n.): rare metallic element, 1832, Modern Latin, named 1828-9 by its discoverer, Swedish chemist Jöns Jakob Berzelius (1779-1848) in honor of the Scandinavian god Thor.
thorn (n.): Old English þorn "sharp point on a stem or branch," earlier "thorny tree or plant," from Proto-Germanic *thurnuz (cf. Old Saxon thorn, Dutch doorn, Old High German dorn, German Dorn, Old Norse þorn, Gothic þaurnus), from PIE *trnus (cf. Old Church Slavonic trunu "thorn," Sanskrit trnam "blade of grass," Greek ternax "stalk of the cactus," Irish trainin "blade of grass"), from *(s)ter-n- "thorny plant," from root *ster- "stiff."

Figurative sense of "anything which causes pain" is recorded from early 13c. (thorn in the flesh is from II Cor. xii:7). Also an Anglo-Saxon and Icelandic runic letter (þ), named for the word of which it was the initial.
thorny (adj.): Old English þornig; see thorn + -y (2). Figurative sense is attested from mid-14c. Related: Thorniness.
thoro: contraction of thorough.
thorough (adj.): late 15c., adjectival use of Old English þuruh (adv.) "from end to end, from side to side," stressed variant of þurh (adv., prep.); see through. Related: thoroughly (c.1300).
thoroughbred (adj.): 1701, of persons, "thoroughly accomplished," from thorough + past tense of breed. In the horse sense of "of pure breed or stock" it dates from 1796; the noun is first recorded 1842.
thoroughfare (n.): late 14c., "passage or way through," from thorough + fare.
thorp (n.): Old English ðorp "village, hamlet, farm, estate," reinforced by Old Norse ðorp, both from Proto-Germanic *thurpa- (cf. Old Frisian thorp, Frisian terp, Middle Dutch, Dutch dorp, German dorf "village," Gothic þaurp "estate, land, field"), probably from PIE root *treb- "dwelling." Preserved in place names ending in -thorp, -thrup.
those: Midlands and southern variant of Old English þas, nominative and accusative plural of þes, þeos "this" (see this).
Thoth: ancient Egyptian god of wisdom and magic, from Latin, from Greek Thoth, from Egyptian Tehuti.
thou (pron.): 2nd nominative singular personal pronoun, Old English þu, from Proto-Germanic *thu (cf. Old Frisian thu, Middle Dutch and Middle Low German du, Old High German and German du, Old Norse þu, Gothic þu), from PIE *tu-, second person singular pronoun (cf. Latin tu, Irish tu, Welsh ti, Greek su, Lithuanian tu, Old Church Slavonic ty, Sanskrit twa-m).

Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. Philadelphia Quakers). The plural at first was used in addressing superior individuals, later also (to err on the side of propriety) strangers, and ultimately all equals. By c.1450 the use of thou to address inferiors gave it a tinge of insult unless addressed by parents to children, or intimates to one another. Hence the verb meaning "to use 'thou' to a person" (mid-15c.).
Avaunt, caitiff, dost thou thou me! I am come of good kin, I tell thee! ["Hickscorner," c.1530]
A brief history of the second person pronoun in English can be found here.
though: c.1200, from Old English þeah, and in part from Old Norse þo "though," both from Proto-Germanic *thaukh (cf. Gothic þauh, Old Frisian thach, Middle Dutch, Dutch doch, Old High German doh, German doch), from PIE demonstrative pronoun *to- (see that). The evolution of the terminal sound did not follow laugh, tough, etc., though a tendency to end the word in "f" existed c.1300-1750 and persists in dialects.
thought (n.): Old English þoht, geþoht, from stem of þencan "to conceive of in the mind, consider" (see think). Cognate with the second element in German Gedächtnis "memory," Andacht "attention, devotion," Bedacht "consideration, deliberation." Second thought "later consideration" is recorded from 1640s. Thought-crime is from "Nineteen Eighty-Four" (1949); thought police is attested from 1946, originally in reference to pre-war Japanese Special Higher Police (Tokubetsu Koto Keisatsu).
thoughtful (adj.): c.1200, "contemplative," from thought + -ful. Also in Middle English, "prudent; moody, anxious." Meaning "showing consideration for others" is from 1851 (cf. thoughtless.) Related: Thoughtfully; thoughtfulness.
thoughtless (adj.): "inconsiderate," 1794, from thought + -less. Related: Thoughtlessly; thoughtlessness.
thousand (n.): Old English þusend, from Proto-Germanic *thusundi (cf. Old Frisian thusend, Dutch duizend, Old High German dusunt, German tausend, Old Norse þusund, Gothic þusundi).

Related to words in Balto-Slavic (cf. Lithuanian tukstantis, Old Church Slavonic tysashta, Polish tysiąc, Czech tisic), and probably ultimately a compound with indefinite meaning "several hundred" or "a great multitude" (with first element perhaps related to Sanskrit tawas "strong, force").

Used to translate Greek khilias, Latin mille, hence the refinement into the precise modern meaning. There was no general Indo-European word for "thousand." Slang shortening thou first recorded 1867. Thousand island dressing (1916) is presumably named for the region of New York on the St. Lawrence River.
thraldom (n.): c.1200; see thrall + -dom.
thrall (n.): Old English þræl "bondman, serf, slave," from Old Norse þræll "slave, servant," probably from Proto-Germanic *thrakhilaz, literally "runner," from root *threh- "to run" (cf. Old High German dregil "servant," prop. "runner;" Old English þrægan, Gothic þragjan "to run").
thrash (v.): 1580s, "to separate grains from wheat, etc., by beating," dialectal variant of threshen (see thresh). Sense of "beat (someone) with (or as if with) a flail" is first recorded c.1600. Meaning "to make wild movements like those of a flail or whip" is attested from 1846. Related: Thrashed; thrashing. Type of fast heavy metal music first called by this name 1982.
thread (n.): Old English þræd "fine cord, especially when twisted" (related to þrawan "to twist"), from Proto-Germanic *thrædus (cf. Middle Dutch draet, Dutch draad, Old High German drat, German Draht, Old Norse þraðr), from suffixed form of root *thræ- "twist" (see throw). Meaning "spiral ridge of a screw" is from 1670s. Threads, slang for "clothes" is 1926, American English.
thread (v.): "to put thread through a needle," mid-14c., from thread (n.); in reference to film cameras from 1913. The dancing move called thread the needle is attested from 1844. Related: Threaded; threading.
threadbare (adj.): mid-14c., from thread (n.) + bare. The notion is of "having the nap worn off," leaving bare the threads.
threat (n.): Old English þreat "crowd, troop," also "oppression, menace," related to þreotan "to trouble, weary," from Proto-Germanic *threutanan (cf. German verdrießen "to vex"), from PIE *trud- "push, press" (cf. Latin trudere "to press, thrust," Old Church Slavonic trudu "oppression," Middle Irish trott "quarrel, conflict," M.Welsh cythrud "torture, torment, afflict"). Sense of "conditional declaration of hostile intention" was in Old English.
threaten (v.): Old English þreatnian (see threat). Related: Threatened. Threatening in the sense of "portending no good" is recorded from 1520s.
three (n.): Old English þreo, fem. and neuter (masc. þri, þrie), from Proto-Germanic *thrijiz (cf. Old Frisian thre, Middle Dutch and Dutch drie, Old High German dri, German drei, Old Norse þrir, Danish tre), from PIE *trejes (cf. Sanskrit trayas, Avestan thri, Greek treis, Latin tres, Lithuanian trys, Old Church Slavonic trye, Irisn and Welsh tri "three").

3-D first attested 1952, abbreviation of three-dimensional (1878). Three-piece suit is recorded from 1909. Three cheers for ______ is recorded from 1751. Three-martini lunch is attested from 1972. Three-ring circus first recorded 1898. Three-sixty "complete turnaround" is from 1927, originally among aviators, in reference to the number of degrees in a full circle. Three musketeers translates French les trois mousquetaires, title of an 1844 novel by Alexandre Dumas père.
threefold (adj.): Old English þrifeald; see three + -fold.
threesome (n.): late 14c., from three + -some.
threnody (n.): "song of lamentation," 1630s, from Greek threnodia, from threnos "dirge, lament" + oide "ode" (see ode). Greek threnos probably is from a PIE imitative root meaning "to murmur, hum;" cf. Old English dran "drone," Gothic drunjus "sound," Greek tenthrene "a kind of wasp."
thresh (v.): Old English þrescan, þerscan "to beat, sift grain by trampling or beating," from Proto-Germanic *threskanan "to thresh," originally "to tread, to stamp noisily" (cf. Middle Dutch derschen, Dutch dorschen, Old High German dreskan, German dreschen, Old Norse þreskja, Gothic þriskan), from PIE root *tere- "to rub, turn" (see throw).

The basic notion is of treading out wheat under foot of men or oxen, later, with the advent of the flail, the word acquired its modern extended sense of "to knock, beat, strike." The original Germanic sense is suggested by the use of the word in Romanic languages that borrowed it, e.g. Italian trescare "to prance," Old French treschier "to dance," Spanish triscar "to stamp the feet."
thresher (n.): late 14c., agent noun from thresh. The thresher shark (c.1600) so called for its long upper tail shape, which resembles a threshing tool.
threshold (n.): Old English þrescold, þærscwold, þerxold "doorsill, point of entering," first element related to Old English þrescan (see thresh), with its original sense of "tread, trample." Second element of unknown origin and much transformed in all the Germanic languages; in English it probably has been altered to conform to hold, but the oft-repeated story that the threshold was a barrier placed at the doorway to hold the chaff flooring in the room is mere folk etymology. Cognates include Old Norse þreskjoldr, Swedish tröskel, Old High German driscufli, German dialectal drischaufel.
threw: past tense of throw (q.v.).
thrice (adv.): c.1200, from Old English þriga, þriwa "thrice" (from þrie "three;" see three) + adverbial genitive -es, changed to -ce c.1600 to reflect voiceless pronunciation.
thrift (n.): c.1300, "fact or condition of thriving," also "prosperity, savings," from Middle English thriven "to thrive" (see thrive), possibly influenced by Old Norse þrift, variant of þrif "prosperity," from þrifask "to thrive." Sense of "habit of saving, economy" first recorded 1550s (thrifty in this sense is recorded from 1520s; also see spendthrift). Thrift shop attested by 1919.
thrifty (adj.): late 14c., from thrift + -y (2). Related: Thriftily; thriftiness.
thrill (v.): c.1300, "to pierce, penetrate," metathesis of Old English þyrlian, from þyrel "hole" (in Middle English, also "nostril"), from þurh "through" (cf. Middle High German dürchel "pierced, perforated;" see through) + -el. Meaning "give a shivering, exciting feeling" is first recorded 1590s, via metaphoric notion of "pierce with emotion." Related: Thrilled; thrilling.
thrill (n.): "a shivering, exciting feeling," 1670s, from thrill (v.). Meaning "a thrilling experience" is attested from 1936.
thriller (n.): 1889, "sensational story," agent noun from thrill (v.).
thrive (v.): c.1200, from Old Norse þrifask "to thrive," originally "grasp to oneself," probably from Old Norse þrifa "to clutch, grasp, grip" (cf. Swedish trifvas, Danish trives "to thrive, flourish"), of unknown origin. Related: Thrived; thriving.
thro: contraction of through.
throat (n.): Old English þrote (implied in þrotbolla "the Adam's apple, larynx," literally "throat boll"), related to þrutian "to swell," from Proto-Germanic *thrut- (cf. Old High German drozza, German Drossel, Old Saxon strota, Middle Dutch strote, Dutch strot "throat"), perhaps from PIE *trud- (cf. Old English þrutian "to swell," Old Norse þrutna "to swell").

The notion is of "the swollen part" of the neck. Italian strozza "throat," strozzare "to strangle" are Germanic loan-words. College slang for "competitive student" is 1970s, from cutthroat.
throb (v.): mid-14c., of uncertain origin, perhaps meant to represent in sound the pulsation of arteries and veins or the heart. Related: Throbbed; throbbing. The noun is first attested 1570s.
throe (n.): c.1200, throwe "pain, pang of childbirth, agony of death," possibly from Old English þrawan "twist, turn, writhe" (see throw), or altered from Old English þrea (genitive þrawe) "affliction, pang, evil, threat" (related to þrowian "to suffer"), from Proto-Germanic *thrawo (cf. Middle High German dro "threat," German drohen "to threaten"). Modern spelling first recorded 1610s. Related: Throes.
thrombo-: word-forming element meaning "blood clot," from comb. form Greek thrombos "clot of blood" (see thrombus).
thrombophlebitis (n.): 1896, from thrombo- + phlebitis.
thrombosis (n.): 1706, Modern Latin, from Greek thrombosis "a clumping or curdling" (from thrombousthai "become curdled or clotted," from thrombos "clot, curd, lump;" see thrombus) + -osis.
thrombus (n.): 1690s, Modern Latin, from Greek thrombos "lump, piece, clot of blood, curd of milk."
throne (n.): mid-13c., from Old French trone (12c.), from Latin thronus, from Greek thronos "elevated seat, chair, throne," from PIE root *dher- (2) "to hold firmly, support" (cf. Latin firmus "firm, steadfast, strong, stable," Sanskrit dharma "statute, law;" see firm (adj.)). Colloquial meaning "toilet" is recorded from 1922.