thesaurus (n.) Look up thesaurus at Dictionary.com
1823, "treasury, storehouse," from Latin thesaurus "treasury, a hoard, a treasure, something laid up," figuratively "repository, collection," from Greek thesauros "a treasure, treasury, storehouse, chest," from root of tithenai "to put, to place" (see theme). The meaning "encyclopedia filled with information" is from 1840, but existed earlier as thesaurarie (1590s), used as a title by early dictionary compilers, on the notion of thesaurus verborum "a treasury of words." Meaning "collection of words arranged according to sense" is first attested 1852 in Roget's title. Thesaurer is attested in Middle English for "treasurer" and thesaur "treasure" was in use 15c.-16c.
these (pron.) Look up these at Dictionary.com
Old English þæs, variant of þas (which became those and took the role of plural of that), nominative and accusative plural of þes, þeos, þis "this" (see this). Differentiation of these and those is from late 13c. OED begins its long entry with the warning, "This word has a complicated history."
Theseus Look up Theseus at Dictionary.com
legendary hero-king of Athens; the name is of uncertain origin.
thesis (n.) Look up thesis at Dictionary.com
late 14c., "unaccented syllable or note," from Latin thesis "unaccented syllable in poetry," later (and more correctly) "stressed part of a metrical foot," from Greek thesis "a proposition," also "downbeat" (in music), originally "a setting down, a placing, an arranging; position, situation," from root of tithenai "to place, put, set," from PIE root *dhe- "to set, to put" (see factitious). Sense in logic of "a formulation in advance of a proposition to be proved" is first recorded 1570s; that of "dissertation presented by a candidate for a university degree" is from 1650s.
thespian (adj.) Look up thespian at Dictionary.com
1670s, "of or pertaining to tragedy or dramatic acting," from Greek Thespis, semi-legendary 6c. B.C.E. poet of Icaria in Attica, often called the Father of Greek Tragedy. The literal meaning of the name is "inspired by the gods."
thespian (n.) Look up thespian at Dictionary.com
"an actor," 1827, from thespian (adj.). Short form thesp is attested from 1962.
Thessaly Look up Thessaly at Dictionary.com
district south of Macedonia and east of Epirus, from Greek Thessalia (Attic Thettalia), an Illyrian name of unknown origin. Related: Thessalian. The city of Thessalonika on the Thermaic Gulf was ancient Therme, renamed when rebuilt by the Macedonian king Cassander, son of Antipater, and named in honor of his wife, Thessalonica, half-sister of Alexander the Great, whose name contains the region name and Greek nike "victory." The adjectival form of it is Thessalonian Related: Thessalonians.
theta (n.) Look up theta at Dictionary.com
eighth letter of the Greek alphabet; in ancient Greece, from Hebrew teth; originally an aspirated -t- (see th). Written on ballots to indicate a vote for a sentence of "death" (thanatos), hence occasional allusive use for "death."
Thetis Look up Thetis at Dictionary.com
name of a sea goddess in Greek mythology, mother of Achilles by Peleus. Since Roman times, sometimes, in poetry, "the sea personified."
theurgy (n.) Look up theurgy at Dictionary.com
1560s, "white magic," from Latin theurgia, from Greek theourgia "sorcery," from theos (genitive theou) "a god" (see theo-) + -ergos "working" (see organ). From 1858 as "the working of divine forces in human affairs." Related: Theurgical.
thew (n.) Look up thew at Dictionary.com
Old English þeaw "usage, custom, habit;" see thews.
thews (n.) Look up thews at Dictionary.com
Old English þeawes "customs, habit, manners; morals, conduct, disposition, personal qualities," plural of þeaw "habit, custom," from Proto-Germanic *thawaz (cognates: Old Saxon thau "usage, custom, habit," Old High German thau "discipline"). According to OED, with no certain cognates outside West Germanic and of unknown origin, but Watkins traces it to PIE root *teue- (1) "to pay attention." Meaning "bodily powers or parts indicating strength, good physique" is attested from 1560s, from notion of "good qualities." Acquired a sense of "muscular development" when it was revived by Scott (1818).
they (pron.) Look up they at Dictionary.com
c.1200, from a Scandinavian source (Old Norse þeir, Old Danish, Old Swedish þer, þair), originally masculine plural demonstrative pronoun, from Proto-Germanic *thai, nominative plural pronoun, from PIE *to-, demonstrative pronoun (see that). Gradually replaced Old English hi, hie, plurals of he, heo "she," hit "it" by c.1400. Colloquial use for "anonymous people in authority" is attested from 1886. They say for "it is said" is in Milton.
The most important importation of this kind [from Scandinavian to English] was that of the pronomial forms they, them and their, which entered readily into the system of English pronouns beginning with the same sound (the, that, this) and were felt to be more distinct than the old native forms which they supplanted. Indeed these were liable to constant confusion with some forms of the singular number (he, him, her) after the vowels has become obscured, so that he and hie, him and heom, her (hire) and heora could no longer be kept easily apart. [Jespersen, "Growth and Structure of the English Language"]
thiamin (n.) Look up thiamin at Dictionary.com
also thiamine, alternative name for vitamin B1, 1937, coined by U.S. chemist Dr. Robert R. Williams (1886-1965) from thio-, indicating the presence of sulfur, comb. form of Greek theion "sulfur," + amine, indicating the amino group. Or the second element might be from vitamin.
thick (adj.) Look up thick at Dictionary.com
Old English þicce "dense, viscous, solid, stiff; numerous, abundant; deep," also as an adverb, "thickly, closely, often, frequently," from Proto-Germanic *thiku- (cognates: Old Saxon thikki, Old High German dicchi, German dick, Old Norse þykkr, Old Frisian thikke), from PIE *tegu- "thick" (cognates: Gaelic tiugh). Secondary Old English sense of "close together" is preserved in thickset and proverbial phrase thick as thieves (1833). Meaning "stupid" is first recorded 1590s. Related: Thickly.

As a noun, "the thick part" (of anything), from mid-13c. Phrase through thick and thin, indicating rough or smooth going, hence "unwaveringly," is in Chaucer (late 14c.); thick-skinned is attested from 1540s; in figurative sense from c.1600. To be in the thick of some action, etc., "to be at the most intense moment" is from 1680s, from a Middle English noun sense.
thicken (v.) Look up thicken at Dictionary.com
late 14c. (transitive), 1590s (intransitive), from thick + -en (1). Related: Thickened; thickening. An earlier verb was Middle English thick, Old English þiccian "to thicken, to crowd together."
thickening (n.) Look up thickening at Dictionary.com
"substance used to thicken something," 1839, verbal noun from thicken.
thicket (n.) Look up thicket at Dictionary.com
"close-set growth of shrubs, bushes, trees, etc.," late Old English þiccet, from þicce (see thick) + denominative suffix -et. Absent in Middle English, reappearing early 16c., perhaps a dialectal survival or a re-formation.
thickness (n.) Look up thickness at Dictionary.com
Old English þicness "density, viscosity, hardness; depth; anything thick or heavy; darkness; thicket;" see thick + -ness.
thickset (adj.) Look up thickset at Dictionary.com
also thick-set, late 14c., thikke sette "with parts or things set close together" (of grass on a sward, etc.), from thick + set (v.). Meaning "stocky, strong and square-built" is recorded from 1724.
thief (n.) Look up thief at Dictionary.com
Old English þeof "thief, robber," from Proto-Germanic *theubaz (cognates: Old Frisian thiaf, Old Saxon thiof, Middle Dutch and Dutch dief, Old High German diob, German dieb, Old Norse þiofr, Gothic þiufs), of uncertain origin.
thieve (v.) Look up thieve at Dictionary.com
Old English þeofian "to thieve, steal," from þeof (see thief). Rare in Old English, rarer in Middle English, not common until 17c.; perhaps the modern word is a late 16c. re-formation. Thieving (adj.) first attested 1520s.
thievery (n.) Look up thievery at Dictionary.com
1560s, from thieve + -ery. An Old English word for it was þeofend.
thievish (adj.) Look up thievish at Dictionary.com
mid-15c., "of or pertaining to thieves," from thieve or thief + -ish. Meaning "inclined to steal" is from 1530s. Wyclif and Chaucer used thiefly (late 14c.). Related: Thievishly; thievishness.
thigh (n.) Look up thigh at Dictionary.com
Old English þeoh, þeh, from Proto-Germanic *theuham (cognates: Old Frisian thiach, Old Dutch thio, Dutch dij, Old Norse þjo, Old High German dioh), probably literally "the thick or fat part of the leg," from PIE *teuk- from root *teue- (2) "to swell" (cognates: Lithuanian taukas, Old Church Slavonic tuku, Russian tuku "fat of animals;" Lithuanian tukti "to become fat;" Avestan tuma "fat;" Greek tylos "callus, lump," tymbos "burial mound, grave, tomb;" Old Irish ton "rump;" Latin tumere "to swell," tumulus "raised heap of earth," tumidus "swollen;"tumor "a swelling;" Middle Irish tomm "a small hill," Welsh tom "mound").
thigmotropism (n.) Look up thigmotropism at Dictionary.com
1900, from thigmo-, comb. form meaning "touch," from Greek thigma "touch" + tropism.
thilk (pron., adj.) Look up thilk at Dictionary.com
"that same, the very thing," early 13c., contraction from þe "the" (see the) + ilce "same" (see ilk).
thimble (n.) Look up thimble at Dictionary.com
Old English þymel "sheath or covering for the thumb," from thuma (see thumb) + -el, suffix used in forming names of instruments (compare handle). Excrescent -b- began mid-15c. (compare humble, nimble). Originally of leather, metal ones came into use 17c. Related: Thimbleful. Thimblerig, con game played with three thimbles and a pea or button, is attested from 1825 by this name, though references to thimble cheats, probably the same swindle, date back to 1716 (see rig (v.)).
thin (adj.) Look up thin at Dictionary.com
Old English þynne "narrow, lean, scanty, not dense; fluid, tenuous; weak, poor," from Proto-Germanic *thunni "thin" (cognates: West Frisian ten, Middle Low German dunne, Middle Dutch dunne, Dutch dun, Old High German dunni, German dünn, Old Norse þunnr, Swedish tunn, Danish tynd), from PIE *tnu- "stretched, stretched out" (hence "thin"), from root *ten- "to stretch" (cognates: Latin tenuis "thin, slender;" see tenet).
These our actors ... were all Spirits, and Are melted into Ayre, into thin Ayre. [Shakespeare, "The Tempest," IV.i.150, 1610]
"Loose or sparse," hence "easily seen through," with figurative extensions. Related: Thinly; thinness. Thin-skinned is attested from 1590s; the figurative sense of "touchy" is from 1670s.
thin (v.) Look up thin at Dictionary.com
Old English þynnian "to make thin, lessen, dilute," also intransitive, "become thin," from thin (adj.). Intransitive sense of "to become less numerous" is attested from 1743; that of "to become thinner" is recorded from 1804. Compare similarly formed German dünnen, Dutch dunnen. Related: Thinned; thinning.
thine (pron.) Look up thine at Dictionary.com
Old English þin, possessive pronoun (originally genitive of þu "thou"), from Proto-Germanic *thinaz (cognates: Old Frisian, Old Saxon thin, Middle Dutch dijn, Old High German din, German dein, Old Norse þin), from PIE *t(w)eino-, suffixed form of second person singular pronomial base *tu-. A brief history of the second person pronoun in English can be found here; see also thou.
thing (n.) Look up thing at Dictionary.com
Old English þing "meeting, assembly, council, discussion," later "entity, being, matter" (subject of deliberation in an assembly), also "act, deed, event, material object, body, being, creature," from Proto-Germanic *thingam "assembly" (cognates: Old Frisian thing "assembly, council, suit, matter, thing," Middle Dutch dinc "court-day, suit, plea, concern, affair, thing," Dutch ding "thing," Old High German ding "public assembly for judgment and business, lawsuit," German ding "affair, matter, thing," Old Norse þing "public assembly"). The Germanic word is perhaps literally "appointed time," from a PIE *tenk- (1), from root *ten- "stretch," perhaps on notion of "stretch of time for a meeting or assembly."

The sense "meeting, assembly" did not survive Old English. For sense evolution, compare French chose, Spanish cosa "thing," from Latin causa "judicial process, lawsuit, case;" Latin res "affair, thing," also "case at law, cause." Old sense is preserved in second element of hustings and in Icelandic Althing, the nation's general assembly.

Of persons, often pityingly, from late 13c. Used colloquially since c.1600 to indicate things the speaker can't name at the moment, often with various meaningless suffixes (see thingamajig). Things "personal possessions" is from c.1300. The thing "what's stylish or fashionable" is recorded from 1762. Phrase do your thing "follow your particular predilection," though associated with hippie-speak of 1960s is attested from 1841.
thingamajig (n.) Look up thingamajig at Dictionary.com
also thingumajig, 1824, see thing. Compare thingum (1670s), thingumbob (1751), thingummy (1796).
think (v.) Look up think at Dictionary.com
Old English þencan "imagine, conceive in the mind; consider, meditate, remember; intend, wish, desire" (past tense þohte, past participle geþoht), probably originally "cause to appear to oneself," from Proto-Germanic *thankjan (cognates: Old Frisian thinka, Old Saxon thenkian, Old High German denchen, German denken, Old Norse þekkja, Gothic þagkjan).

Old English þencan is the causative form of the distinct Old English verb þyncan "to seem, to appear" (past tense þuhte, past participle geþuht), from Proto-Germanic *thunkjan (cognates: German dünken, däuchte). Both are from PIE *tong- "to think, feel" which also is the root of thought and thank.

The two Old English words converged in Middle English and þyncan "to seem" was absorbed, except for its preservation in archaic methinks "it seems to me." As a noun, "act of prolonged thinking," from 1834. The figurative thinking cap is attested from 1839.
think tank (n.) Look up think tank at Dictionary.com
also think-tank, 1959 as "research institute" (first reference is to Center for Behavioral Sciences, Palo Alto, Calif.); it had been colloquial for "the brain" since 1905. See think + tank (n.).
thinkable (adj.) Look up thinkable at Dictionary.com
1805; see think (v.) + -able. Possibly a back-formation from unthinkable.
thinker (n.) Look up thinker at Dictionary.com
"one who has cultivated the powers of thought," mid-15c., agent noun from think (v.).
thinner (n.) Look up thinner at Dictionary.com
liquid used to dilute paint, ink, etc., 1904, agent noun from thin (v.).
third (adj.) Look up third at Dictionary.com
late Old English metathesis of þridda, from Proto-Germanic *thridja- (cognates: Old Frisian thredda, Old Saxon thriddio, Middle Low German drudde, Dutch derde, Old High German dritto, German dritte, Old Norse þriðe, Danish tredie, Swedish tredje, Gothic þridja), from PIE *tri-tyo- (cognates: Sanskrit trtiyas, Avestan thritya, Greek tritos, Latin tertius (source of Italian terzo, Spanish tercio, French tiers), Old Church Slavonic tretiji, Lithuanian trecias, Old Irish triss, Welsh tryde), suffixed form of root *trei- (see three).

Metathesis of thrid into third is attested from c.950 in Northumbrian, but overall thrid was prevalent up to 16c. The noun meaning "third part of anything" is recorded from late 14c. Third rail in electric railway sense is recorded from 1890. Third World War as a possibility first recorded 1947. Third-rate "of poor quality" is from 1814, ultimately from classification of ships (1640s); third class in railway travel is from 1839. Third Reich (1930) is a partial translation of German drittes Reich (1923). Third party in law, insurance, etc., is from 1818.
third degree (n.) Look up third degree at Dictionary.com
"intense interrogation by police," 1900, probably a reference to Third Degree of master mason in Freemasonry (1772), the conferring of which included an interrogation ceremony. Third degree as a measure of severity of burns (most severe) is attested from 1866, from French (1832); in American English, as a definition of the seriousness of a particular type of crime (the least serious type) it is recorded from 1865.
Third World (n.) Look up Third World at Dictionary.com
1963, from French tiers monde, formulated 1952 by French economic historian Alfred Sauvy (1898-1990) on model of the third estate (French tiers état) of Revolutionary France; his first world (The West) and second world (the Soviet bloc) never caught on.
thirst (v.) Look up thirst at Dictionary.com
Old English þyrstan "to thirst, thirst after," from the noun (see thirst (n.)); the figurative sense of the verb was present in Old English. Compare Old Saxon thurstian, Dutch dorsten, Old High German dursten, German dürsten, all verbs from nouns. Related: Thirsted; thirsting.
thirst (n.) Look up thirst at Dictionary.com
Old English þurst, from Proto-Germanic *thurstu- (cognates: Old Saxon thurst, Frisian torst, Dutch dorst, Old High German and German durst), from Proto-Germanic verbal stem *thurs- (cognates: Gothic thaursjan, Old English thyrre), from PIE root *ters- "dry" (see terrain). Figurative sense of "vehement desire" is attested from c.1200.
thirsty (adj.) Look up thirsty at Dictionary.com
Old English þurstig "thirsty, greedy;" see thirst (n.) + -y (2). Related: Thirstily; thirstiness. Similar formation in Old Frisian, Dutch dorstig, German durstig.
thirteen (adj.) Look up thirteen at Dictionary.com
late 145c., metathesis of Middle English thrittene, from Old English þreotene (Mercian), þreotiene (West Saxon), from þreo "three" (see three) + -tene (see -teen). Similar formation in Old Saxon thriutein, Old Frisian thretten, Dutch dertien, German dreizehn, Old Norse threttan, Swedish tretton. As a noun from late Old English.

Not an unlucky number in medieval England, but associated rather with the customary "extra item" (as in baker's dozen). Superstitions began with association with the Last Supper, and the unluckiness of 13 sitting down together to dine (attested from 1690s). Most of the modern superstitions (buildings with floor "12-A," etc.) have developed since 1890.
thirteenth (adj.) Look up thirteenth at Dictionary.com
15c. metathesis of þriteenþe (mid-14c.; see thirteen + -th (1)), which replaced forms from Old English þreoteoða (West Saxon), þriteogeða (Anglian). Similar formation in Old Norse þrettande, Danish trettende, Swedish trettonde, Old Frisian threttinde, Dutch dertiende, Old High German dritto-zehanto, German dreizehnte.
thirties (n.) Look up thirties at Dictionary.com
1827 as the years of someone's life between 30 and 39; 1830 as the fourth decade of years in a given century. See thirty.
thirtieth Look up thirtieth at Dictionary.com
Middle English threttyth, from Old English þritigoða; see thirty + -th (1). Respelled 16c. to conform to new spelling of thirty.
thirty (adj.) Look up thirty at Dictionary.com
late 14c. metathesis of thritti, from Old English þritig, from þri, þreo "three" (see three) + -tig "group of ten" (see -ty (1)). Similar formation in Old Frisian thritich, Old Saxon thritig, Dutch dertig, Old High German drizzug, German dreissig.

The Thirty Years' War (1842) was a religious power struggle waged 1618-48, mainly on German soil. The symbol -30- as printer and telegrapher's code to indicate the last sheet or line of copy or a dispatch is recorded from 1895. In 20c. jargon of newspaper journalism, it came to be a traditional sign-off signal and slang word for "the end."
this (pron.) Look up this at Dictionary.com
Old English þis, neuter demonstrative pronoun and adjective (masc. þes, fem. þeos), probably from a North Sea Germanic pronoun *tha-si-, formed by combining the base *þa- (see that) with -s, which is probably identical with Old English se "the" (representing here "a specific thing"), or with Old English seo, imperative of see (v.) "to behold." Compare Old Saxon these, Old Frisian this, Old Norse þessi, Middle Dutch dese, Dutch deze, Old High German deser, German dieser.

Once fully inflected, with 10 distinct forms (see table below); the oblique cases and other genders gradually fell away by 15c. The Old English plural was þæs (nominative and accusative), which in Northern Middle English became thas, and in Midlands and Southern England became thos. The Southern form began to be used late 13c. as the plural of that (replacing Middle English tho, from Old English þa) and acquired an -e (apparently from the influence of Middle English adjective plurals in -e; compare alle from all, summe from sum "some"), emerging early 14c. as modern those.

About 1175 thes (probably a variant of Old English þæs) began to be used as the plural of this, and by 1200 it had taken the form these, the final -e acquired via the same mechanism that gave one to those.

Masc.Fem.Neut.Plural
Nom.þesþeosþisþas
Acc.þisneþasþisþas
Gen.þissesþisseþissesþissa
Dat.þissumþisseþissumþissum
Inst.þysþisseþysþissum