Etymology
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their (pron.)
plural possessive pronoun, c. 1200, from Old Norse þierra "of them," genitive of plural personal and demonstrative pronoun þeir "they" (see they). Replaced Old English hiera. As an adjective from late 14c. Use with singular objects, scorned by grammarians, is attested from c. 1300, and OED quotes this in Fielding, Goldsmith, Sydney Smith, and Thackeray. Theirs (c. 1300) is a double possessive. Alternative form theirn (1836) is attested in Midlands and southern dialect in U.K. and the Ozarks region of the U.S.
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theist (n.)
1660s, from Greek theos "god" (from PIE root *dhes-, forming words for religious concepts) + -ist. The original senses was that later reserved to deist: "one who believes in a transcendent god but denies revelation." Later in 18c. theist was contrasted with deist, as believing in a personal God and allowing the possibility of revelation.
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theme (n.)
early 14c., "subject or topic on which a person writes or speaks," from Old French tesme (13c., with silent -s- "indicating vowel length" [OED], Modern French thème) and directly from Latin thema "a subject, thesis," from Greek thema "a proposition, subject, deposit," literally "something set down," from PIE *dhe-mn, suffixed form of root *dhe- "to set, put." Meaning "school essay" is from 1540s. Extension to music first recorded 1670s; theme song first attested 1929. Theme park is from 1960.
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Themis 
Greek goddess of law and justice, the name means "custom, right," literally "that which is laid down or established" (by custom); also "laws, ordinances," but closer in sense to Latin ius (see jurist) than to lex (see legal); related to thema "proposition; that which is placed" (see theme).
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theory (n.)

1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theōria "contemplation, speculation; a looking at, viewing; a sight, show, spectacle, things looked at," from theōrein "to consider, speculate, look at," from theōros "spectator," from thea "a view" (see theater) + horan "to see," which is possibly from PIE root *wer- (3) "to perceive."

Earlier in this sense was theorical (n.), late 15c. Sense of "principles or methods of a science or art" (rather than its practice) is first recorded 1610s (as in music theory, which is the science of musical composition, apart from practice or performance). Sense of "an intelligible explanation based on observation and reasoning" is from 1630s.

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therapeutic (adj.)
pertaining to the healing of disease, 1640s, from Modern Latin therapeuticus "curing, healing," from Greek therapeutikos, from therapeutein "to cure, treat medically," primarily "do service, take care of, provide for," of unknown origin, related to therapon "attendant." Therapeutic was used from 1540s as a noun meaning "the branch of medicine concerned with treatment of disease." Related: Therapeutical (c. 1600).
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thermometer (n.)
1630s, from French thermomètre (1620s), coined by Jesuit Father Jean Leuréchon from Greek thermos "hot" (see thermal) + metron "measure" (from PIE root *me- (2) "to measure"). 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.
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Thermos (n.)
trademark registered in Britain 1907, invented by Sir James Dewar (patented 1904 but not named then), from Greek thermos "hot" (from PIE root *gwher- "to heat, warm"). 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.
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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. 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).
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Themistocles 
name of great Athenian political leader, from Greek Themistokles, literally "famed in law and right," from themis "custom, law, right" (see Themis) + -kles "fame," a common ending in Greek proper names, related to kleos "rumor, report, news; good report, fame, glory," from PIE *klew-yo-, suffixed form of root *kleu- "to hear."
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