Etymology
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thereafter (adv.)
Old English þær æfter; see there + after. Similar formation in Dutch daarachter, Swedish derefter.
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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 daarvoor, German dafür, Danish derfor.

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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.
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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].
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theatrical (adj.)
1550s, "pertaining to the theater;" see theater + -ical. Sense of "stagy, histrionic" is attested from 1709. Related: Theatrically; theatricality.
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thermocline (n.)
1897, from thermo- + -cline, from Greek klinein "to slope," from PIE root *klei- "to lean."
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theodolite (n.)
surveying instrument, 1570s, of unknown origin (see OED for discussion). "The word has a Gr[eek] semblance, but no obvious Gr[eek] basis" [Century Dictionary].
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theirs (pron.)
possessive pronoun, "their own," early 14c., from their + possessive -s, on analogy of his, etc. In form, a double possessive.
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