Etymology
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horse-chestnut (n.)
1590s, from horse (n.) + chestnut. A tree probably native to Asia, introduced in England c. 1550; the name also was extended to similar North American species such as the buckeye. Said to have been so called because it was food for horses, "but this is appar. a mere guess" [Century Dictionary] and the sense is perhaps "large," as in horseradish. The nut resembles that of the edible chestnut but is bitter to the taste.
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sea-horse (n.)

late 15c., "walrus" (apparently), from sea + horse (n.); compare walrus. Also in heraldry as a fabulous animal with the foreparts of a horse and the tail of a fish. Main modern sense in zoology is attested from 1580s.

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horse-shit (n.)
also horse-shit, by 1935, from horse (n.) + shit (n.).
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horse-marine (n.)
1824, "one of an imaginary corps of mounted sailors," hence "a person out of his element and unfit for his place" [Century Dictionary], from horse (n.) + marine (n.). However by 1878 the term was being used in fact in reference to cavalrymen pressed into marine service or seamen mounted as an improvised shore defense.
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post-horse (n.)

horse kept at an inn, post house, or other station for use by mail carriers or for rent to travelers, 1520s, from post (n.3) "communication from one place to another by relays" + horse (n.).

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one-horse (adj.)

"small-scale, petty" 1853, American English, colloquial, in reference to towns; see one + horse (n.). Probably from earlier use in reference to a carriage, sleigh, plow, etc., "drawn by a single horse" (1750); also "possessing only one horse" (of a farmer); hence "petty, on a small scale, of limited capacity or resources; inferior."

Shortly afterwards I took a stroll over the town. It was what is generally denominated a "one horse town," and I would think a pretty small pony at that. Two stores, one grocery, a stable, and four dwellings made up the sum of its buildings. ["Daguerreotyping in the Back Woods," in Yankee Notions, March, 1855]
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horse-play (n.)
also horseplay, "rough, excessive play," 1580s, from horse (n.) with its associations of "strong, coarse" + play (n.).
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aurora (n.)

"morning light, dawn," late 14c., from Latin Aurora, the Roman goddess of dawn, from PIE *ausus- "dawn," also the name of the Indo-European goddess of the dawn, from root *aus- (1) "to shine," especially of the dawn (source also of Greek ēōs "dawn").

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aubade (n.)
"song to be performed in open air in the early morning, musical announcement of dawn," 1670s, from French aubade "dawn" (15c.), from Provençal aubada, from auba "dawn," from Latin alba, fem. of albus "white" (see alb).
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*aus- (1)

Proto-Indo-European root meaning "to shine," especially of the dawn. It forms all or part of: austral; Australia; Austria; Austro-; Aurora; east; Easter; eastern; eo-; Ostrogoth.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit usah "dawn;" Greek ēōs "dawn;" Latin Aurora "goddess of dawn," auster "south wind;" Lithuanian aušra "dawn;" Old English east "east."

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