Online Etymology Dictionary

there (adv., conj.): Old English þær "in or at that place, so far as, provided that, in that respect," from Proto-Germanic *thær (source also of Old Saxon thar, Old Frisian ther, Middle Low German dar, Middle Dutch daer, Dutch daar, Old High German dar, German da, Gothic þar, Old Norse þar), from PIE *tar- "there" (source also of Sanskrit tar-hi "then"), from root *to- (see the) + adverbial suffix -r.

Interjectional use is recorded from 1530s, used variously to emphasize certainty, encouragement, or consolation. To have been there "had previous experience of some activity" is recorded from 1877.
thereabouts (adv.): early 15c., "in that area, around there; mid-15c., "near to that time, approximately thence," from Old English þær onbutan "about that place" + adverbial genitive -es; see there + about.
thereafter (adv.): Old English þær æfter; see there + after. Similar formation in Dutch daarachter, Swedish derefter.
thereby (adv.): Old English þærbig "thus, by means of or because of that;" see there + by. Similar formation in Old Frisian therbi, Middle Low German darbi, German dabei, Dutch daarbij.
therefor (adv.): "for this, for that," Middle English variant spelling of therefore (q.v.); in modern use perhaps perceived as there + for.
therefore (adv.): Old English þærfore; from there + fore, Old English and Middle English collateral form of for. Since c. 1800, therefor has been used in sense of "for that, by reason of that;" and therefore in sense of "in consequence of that." Similar formation in Dutch daarfoor, German dafür, Danish derfor.
therefrom (adv.): mid-13c., there from. One word from 17c.; see there + from.
therein (adv.): "in that place, time, or thing," Old English þærin; see there + in. Similar formation in German darin.
theremin (n.): 1927, from the name of its inventor, Russian engineer Léon Thérémin (1896-1993).
thereof (adv.): "of that, of it," Old English þærof; see there + of. Similar formation in Swedish, Danish deraf.
thereon (adv.): Old English þæron; see there + on. Similar formation in German daran.
Theresa: also Teresa, fem. proper name, from French Thérèse, from Latin Therasia, apparently from Greek Therasia, name of two volcanic islands, one near Sicily, one near Crete. In the top 50 most popular names for girls born in the U.S. from 1953 to 1969.
thereto (adv.): Old English þærto "to it, in that place, for that purpose, belonging to;" see there + to. Similar formation in Old Saxon tharto, Old High German darazuo, German dazu.
thereunder (adv.): Old English þærunder; see there + under. Similar formation in Old Frisian therunder, German darunter.
thereupon (adv.): late 12c., þer uppon; see there + upon.
therewith (adv.): c. 1200, "along with, in company with," from there + with. Old English þær wiþ meant "against, in exchange for." Similar formation in Swedish dervid, Danish derved.
thermal (adj.): 1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat, feverish heat," from PIE *gwher- "to heat, warm" (source also of Latin fornax "an oven, kiln," formus "warm," Old English wearm; see warm (adj.)). Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.
thermo-: before vowels therm-, word-forming element meaning "hot, heat, temperature," used in scientific and technical words, from comb. form of Greek thermos "hot, warm," therme "heat" (see thermal).
thermochemistry (n.): also thermo-chemistry, 1840, from thermo- + chemistry.
thermocline (n.): 1897, from thermo- + -cline, from Greek klinein "to slope" (see lean (v.)).
thermocouple (n.): also thermo-couple, 1862, from thermo-electric + couple (n.).
thermodynamic (adj.): 1849, from thermo- + dynamic (adj.).
thermodynamics (n.): theory of relationship between heat and mechanical energy, 1854, from thermodynamic (adj.); also see -ics. "The consideration of moving forces, though suggested by the form of the word, does not enter into the subject to any considerable extent" [Century Dictionary].
thermograph (n.): "automatic self-registering thermometer," 1881, from thermo- + -graph "instrument for recording; something written." Related: Thermographic.
thermography (n.): 1840, "method of writing which requires heat to develop the characters," from thermo- + -graphy.
thermometer (n.): 1630s, from French thermomètre (1620s), coined by Jesuit Father Jean Leuréchon from Greek thermos "hot" (see thermal) + metron "measure" (see meter (n.2)). An earlier, Latinate form was thermoscopium (1610s). The earliest such device was Galileo's air-thermometer, invented c. 1597. The typical modern version, with mercury in glass, was invented by Fahrenheit in 1714. Related: Thermometric; thermometrical.
thermonuclear (adj.): 1938 with reference to stars, 1953 of weapons (technically only to describe the hydrogen bomb), from thermo- + nuclear.
thermoplastic (adj.): 1870, see thermo- + plastic (adj.). As a noun from 1929.
Thermopylae: narrow land passage along the Malian Gulf in ancient Greece, from Greek thermos "hot" (see thermal) + pylai, plural of pyle "gate; mountain pass, entrance into a region" (see pylon). In reference to nearby hot sulfur springs. Often simply hai pylai "the gates." Figurative of heroic resistance against overwhelming numbers since the battle fought there between the Greeks and Persians in 480 B.C.E.
Thermos (n.): trademark registered in Britain 1907, invented by Sir James Dewar (patented 1904 but not named then), from Greek thermos "hot" (see thermal). Dewar built the first one in 1892, but it was first manufactured commercially in Germany in 1904, when two glass blowers formed Thermos GmbH. Supposedly the company sponsored a contest to name the thing, and a Munich resident won with a submission of Thermos.
thermosphere (n.): 1924, from thermo- + sphere.
thermostat (n.): automatic instrument for regulating temperature, 1831, from thermo- + -stat.
Theropoda (n.): order of dinosaurs, Modern Latin, from Greek elements ther- "beast" + podos genitive of pous "foot," from PIE root *ped- (1) "a foot" (see foot (n.)). So called because the structure of the feet resembled quadrupeds rather than birds. Related: Theropod.
thesaurus (n.): 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.): 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: legendary hero-king of Athens; the name is of uncertain origin.
thesis (n.): 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.): 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.): "an actor," 1827, from thespian (adj.). Short form thesp is attested from 1962.
Thessaly: 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.): 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: name of a sea goddess in Greek mythology, mother of Achilles by Peleus. Since Roman times, sometimes, in poetry, "the sea personified."
theurgy (n.): 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.): Old English þeaw "usage, custom, habit;" see thews.
thews (n.): Old English þeawes "customs, habit, manners; morals, conduct, disposition, personal qualities," plural of þeaw "habit, custom," from Proto-Germanic *thawaz (source also of 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.): 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.): 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.): Old English þicce "dense, viscous, solid, stiff; numerous, abundant; deep," also as an adverb, "thickly, closely, often, frequently," from Proto-Germanic *thiku- (source also of Old Saxon thikki, Old High German dicchi, German dick, Old Norse þykkr, Old Frisian thikke), from PIE *tegu- "thick" (source also of 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.): 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.): "substance used to thicken something," 1839, verbal noun from thicken.