Online Etymology Dictionary

thorny (adj.): Old English þornig; see thorn + -y (2). Figurative sense is attested from mid-14c. Related: Thorniness. Similar formation in Dutch doornig, German dornig.
thoro: abbreviated spelling of thorough.
thorough (adj.): c. 1300, 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; thoroughness.
thoroughbred (adj.): 1701, of persons, "thoroughly accomplished," from thorough + past tense of breed. In reference to horses, "of pure breed or stock," from 1796; the noun is first recorded 1842.
thoroughfare (n.): late 14c., "passage or way through," from thorough (before it had differentiated from through) + fare (n.).
thoroughgoing (adj.): 1800, from thorough + going.
thorp (n.): Old English ðorp "village, hamlet, farm, estate," reinforced by Old Norse ðorp, both from Proto-Germanic *thurpa- (source also of Old Frisian thorp, Frisian terp, Middle Dutch, Dutch dorp, German dorf "village," Gothic þaurp "estate, land, field"), probably from PIE root *treb- "dwelling" (see tavern). Preserved in place names ending in -thorp, -thrup.
those (pron.): c. 1300, Midlands and southern variant of Old English þas, nominative and accusative plural of þes, þeos "this" (see this). A collateral form of these, now used as the plural of that.
Thoth: ancient Egyptian god of wisdom and magic, hieroglyphics, and the reckoning of time, from Latin, from Greek Thoth, from Egyptian Tehuti. Usually represented as a human figure with the head of an ibis. By the Greeks, assimilated to their Hermes.
thou (pron.): 2nd nominative singular personal pronoun, Old English þu, from Proto-Germanic *thu (source also of 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 (source also of 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Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. early Quakers).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 (adv., conj.): c. 1200, from Old English þeah "though, although, even if, however, nevertheless, although, still, yet;" and in part from Old Norse þo "though," both from Proto-Germanic *thaukh (source also of 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 "process of thinking, a thought; compassion," 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.

Bammesberger ("English Etymology") explains that in Germanic -kt- generally shifted to -ht-, and a nasal before -ht- was lost. Proto-Germanic *thankija- added a suffix -t in the past tense. By the first pattern the Germanic form was *thanht-, by the second the Old English was þoht.

Thought-crime is from "Nineteen Eighty-Four" (1949); thought police is attested from 1945, originally in reference to war-time Japanese Special Higher Police (Tokubetsu Koto Keisatsu).
thoughtful (adj.): c. 1200, "contemplative, occupied with thought," from thought + -ful. Also in Middle English, "prudent; moody, anxious." Meaning "showing consideration for others" is from 1851 (compare thoughtless.) Related: Thoughtfully; thoughtfulness.
thoughtless (adj.): 1610s, "heedless, imprudent," from thought + -less. Meaning "inconsiderate of others" is from 1794. Related: Thoughtlessly; thoughtlessness.
thousand (adj.): Old English þusend, from Proto-Germanic *thusundi (source also of Old Frisian thusend, Dutch duizend, Old High German dusunt, German tausend, Old Norse þusund, Gothic þusundi).

Related to words in Balto-Slavic (Lithuanian tukstantis, Old Church Slavonic tysashta, Polish tysiąc, Russian tysiacha, Czech tisic), and probably ultimately a compound with indefinite meaning "great multitude, several hundred," literally "swollen-hundred," with first element from PIE root *teue- (2) "to swell" (see thigh).

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) presumably is named for the region of New York on the St. Lawrence River.
thousandth (adj.): 1550s, from thousand + -th (1).
Thrace: Greek Thrake, named for the people who inhabited it, whose name is of unknown origin, perhaps Semitic. Related: Thracian.
thraldom (n.): also thralldom, c. 1200; see thrall + -dom.
thrall (n.): late Old English þræl "bondman, serf, slave," from a Scandinavian source akin to Old Norse þræll "slave, servant," figuratively "wretch, scoundrel," probably from Proto-Germanic *thrakhilaz, literally "runner," from root *threh- "to run" (source also of Old High German dregil "servant," properly "runner;" Old English þrægan, Gothic þragjan "to run"). Meaning "condition of servitude" is from early 14c.
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 1620s. Meaning "to make wild movements like those of a flail or whip" is attested from 1846. Related: Thrashed; thrashing. As a noun from 1660s, "threshing tool;" 1840s as "a beating;" 1982 as the name for a type of fast heavy metal music.
thread (n.): Old English þræd "fine cord, especially when twisted" (related to þrawan "to twist"), from Proto-Germanic *thredu- "twisted yarn" (source also of Old Saxon thrad, Old Frisian thred, Middle Dutch draet, Dutch draad, Old High German drat, German Draht, Old Norse þraðr), literally "twisted," from suffixed form of PIE root *tere- (1) "to rub, rub by turning, turn" (see throw (v.)). Meaning "spiral ridge of a screw" is from 1670s. Threads, slang for "clothes" is 1926, American English.
The silk line, as spun by the worm, is about the 5000th part of an inch thick; but a spider's line is perhaps six times finer, or only the 30,000th part of an inch in diameter, insomuch, that a single pound of this attenuated substance might be sufficient to encompass our globe. [John Leslie, "Elements of Natural Philosophy," Edinburgh, 1823]
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.): late 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, coercion, menace," related to þreotan "to trouble, weary," from Proto-Germanic *thrautam (source also of Dutch verdrieten, German verdrießen "to vex"), from PIE *treud- "to push, press squeeze" (source also of Latin trudere "to press, thrust," Old Church Slavonic trudu "oppression," Middle Irish trott "quarrel, conflict," Middle Welsh cythrud "torture, torment, afflict"). Sense of "conditional declaration of hostile intention" was in Old English.
threaten (v.): late 13c., "attempt to influence by menacing," from Old English þreatnian "to threaten" (see threat). Related: Threatened. Threatening in the sense of "portending no good" is recorded from 1520s.
three (adj.): Old English þreo, fem. and neuter (masc. þri, þrie), from Proto-Germanic *thrijiz (source also of Old Saxon thria, Old Frisian thre, Middle Dutch and Dutch drie, Old High German dri, German drei, Old Norse þrir, Danish tre), from nominative plural of PIE root *trei- "three" (source also of Sanskrit trayas, Avestan thri, Greek treis, Latin tres, Lithuanian trys, Old Church Slavonic trye, Irish 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 the 1844 novel by Alexandre Dumas père.
Three Rs (n.): 1824; said to have been given as a toast by Sir William Curtis (1752-1829), a beloved lord mayor of London in the 1820s, who seems to have been a figure of fun to whom many mangled phrases were attributed. Among the toasts he is alleged to have given at public dinners were "The Female Ladies of London;" "The three C's--Cox, King, and Curtis;" and "The three R's--Reading, Writing, and Rithmetic."
It has been very much the fashion amongst a class of persons to attribute to Sir W. C. ... a vulgarity and ignorance of speech which are by no means consistent with his character and conduct. The worthy and hospitable baronet has a rapid mode of speech, but it is always correct ; and although some eccentricities are mixed up in his composition, he is highly honourable, and has been a very useful member of society, particularly to his London constituents. ["The Mirror," Jan. 29, 1825]
After listing some examples, the article continues:
It is, however, very certain, that at a city festival some years ago, having indulged very freely, he fell asleep, when some wag, choosing to consider him dead, wrote his epitaph, which was found next morning pinned to the baronet's dress coat:--

"Here lies the great Curtis,
Of London, Lord May'r:
He's left this here world,
And gone to that there."
threefold (adj.): late Old English þrifeald; see three + -fold.
threesome (n.): late 14c., from three + -some (2).
threnody (n.): "song of lamentation," 1630s, from Greek threnodia "lamentation," from threnos "dirge, lament" + oide "ode" (see ode). Greek threnos probably is from PIE imitative root *dher- (3) "to drone, murmur, hum;" source also of 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 *threskan "to thresh," originally "to tread, to stamp noisily" (source also of Middle Dutch derschen, Dutch dorschen, Old High German dreskan, German dreschen, Old Norse þreskja, Swedish tröska, Gothic þriskan), from PIE root *tere- (1) "to rub, turn" (see throw (v.)).

The basic notion is of men or oxen treading out wheat; 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, such as 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, which resembles a threshing tool.
threshold (n.): Old English þrescold, þærscwold, þerxold, etc., "door-sill, point of entering," of uncertain origin and probably much altered by folk-etymology. The first element probably is related to Old English þrescan (see thresh), either in its current sense of "thresh" or with its original sense of "tread, trample." Second element has been much transformed in all the Germanic languages, suggesting its literal sense was lost even in ancient times. In English it probably has been altered to conform to hold. Liberman (Oxford University Press blog, Feb. 11, 2015) revives an old theory that the second element is the Proto-Germanic instrumental suffix *-thlo and the original sense of threshold was a threshing area adjacent to the living area of a house. Cognates include Old Norse þreskjoldr, Swedish tröskel, Old High German driscufli, German dialectal drischaufel. Figurative use was in Old English.
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 c. 1600 to -ce to reflect voiceless pronunciation.
thrift (n.): c. 1300, "fact or condition of thriving," also "prosperity, savings," from Middle English thriven "to thrive" (see thrive), influenced by (or from) 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., "respectable," from thrift + -y (2). Meaning "frugal" is from 1520s. Related: Thriftily; thriftiness.
thrill (v.): early 14c., "to pierce, penetrate," metathesis of Old English þyrlian "to perforate, pierce," from þyrel "hole" (in Middle English, also "nostril"), from þurh "through" (compare 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 a Scandinavian source akin to Old Norse þrifask "to thrive," originally "grasp to oneself," probably reflexive of þrifa "to clutch, grasp, grip, take hold of" (compare Norwegian triva "to seize," Swedish trifvas, Danish trives "to thrive, flourish"), of unknown origin. Related: Thrived (or throve); thriving.
thro (prep.): shorter spelling 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- (source also of Old High German drozza, German Drossel, Old Saxon strota, Middle Dutch strote, Dutch strot "throat"), of uncertain origin. Italian strozza "throat," strozzare "to strangle" are Germanic loan-words. College slang for "competitive student" is 1970s, from cutthroat.
throaty (adj.): 1640s, from throat + -y (2). Related: Throatily; throatiness.
throb (v.): late 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," of uncertain origin, possibly from Old English þrawan "twist, turn, writhe" (see throw (v.)), or altered from Old English þrea (genitive þrawe) "affliction, pang, evil; threat, persecution" (related to þrowian "to suffer"), from Proto-Germanic *thrawo (source also of Middle High German dro "threat," German drohen "to threaten"). Modern spelling first recorded 1610s. Related: Throes.
thrombo-: before vowels thromb-, word-forming element meaning "blood clot," from comb. form Greek thrombos "clot of blood" (see thrombus).
thrombophlebitis (n.): 1872, 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."