Gk. hippos, L. equus, O.Ir. ech, Goth. aihwa-, Skt. açva-, all meaning "horse"). In many other languages, as in O.E., this root has been lost in favor of synonyms, probably via superstitious taboo on uttering the name of an animal so important in I.E. religion. Used since at least 1391 of various devices or appliances which suggest a horse (e.g. sawhorse). To ride a horse that was foaled of an acorn (1678) was through early 19c. a way to say "be hanged from the gallows." Slang for heroin is first attested 1950. Horseplay is from 1589. The belief that finding a horseshoe by chance is lucky is attested from late 14c. Horse latitudes first attested 1777, the name of unknown origin, despite much speculation. Dead horse as a figure for "something that has ceased to be useful" is attested from 1638. High horse originally (c.1380) was "war horse, charger;" fig. sense in mount (one's) high horse "affect airs of superiority" is from 1782. The horse's mouth as a source of reliable information is from 1928, perhaps from the fact that a horse's age can be determined accurately by looking at its teeth. To swap horses while crossing the river (a bad idea) is first attested 1864 in writings of Abraham Lincoln. Horse sense is 1870, Amer.Eng. colloquial, probably from the same association of "strong, large, coarse" found in horseradish
. Horse and buggy meaning "old-fashioned" is recorded from 1926 slang, originally in ref. to a "young lady out of date, with long hair." The proverbial gift horse was earlier given horse:
"No man ought to looke a geuen hors in the mouth." [Heywood, 1546]
The modern form perhaps traces to Butler's "Hudibras" (1663), where the tight iambic tetrameter required a shorter phrase:
He ne'er consider'd it, as loth
To look a Gift-horse in the mouth.