Etymology
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pistol (n.)

"small firearm with a curved handle, intended to be held in one hand when aimed and fired," 1570s, from French pistole "short firearm" (1566), a word of uncertain origin, sometimes said to be from German Pistole, from Czech pis'tala "firearm," literally "tube, pipe," from pisteti "to whistle," a word of imitative origin, related to Russian pischal "shepherd's pipe."

But the earlier English form pistolet (1550) is said to be from French pistolet "a small firearm," also "a small dagger," which is said to be connected with Italian pistolese, in reference to Pistoia, the town in Tuscany noted for gunsmithing.

Pistol-whip (v.) "strike (someone) with the butt of a pistol is recorded by 1942. Pistol-grip "handle shaped like the butt of a pistol" is by 1874.

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horse (v.)

late Old English horsian "to provide with a horse or horses," from horse (n.). Related: Horsed; horsing. Sense of "to play excessive jokes on" is by 1893, mostly in formation horse around (1928), perhaps from horse-play, or from earlier nautical jargon use of the verb in reference to men, "drive or urge to work unfairly and tyrannically" (1867). But also consider the vulgar expressions arsing about (1660s), arsing around (1922).

[A] favorite pastime for many men is to "horse" or guy a friend who has shown himself susceptible to ridicule or fun making. "Horsing" is extremely wholesome mental discipline for over sensitive or super-conceited young men. "Horsing" always implies a joke at another's expense. As to how it came into use there is no satisfactory theory to offer. [Yale Literary Magazine, December 1893]

As a verb, horse also meant "to mount on horseback" (early 14c., horsen), "to spank" as one does a horse to get it to go (1825), also "to copulate, mount" (as a stallion does a mare), hence figuratively, of men, "copulate with" a woman (mid-15c.).

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horse (n.)

"solidungulate perissodactyl mammal of the family Equidæ and genus Equus" [Century Dictionary], Old English hors "horse," from Proto-Germanic *harss- (source also of Old Norse hross, Old Frisian, Old Saxon hors, Middle Dutch ors, Dutch ros, Old High German hros, German Roß "horse"), of unknown origin. By some, connected to PIE root *kers- "to run," source of Latin currere "to run." Boutkan prefers the theory that it is a loan-word from an Iranian language (Sarmatian) also borrowed into Uralic (compare Finnish varsa "foal"),

The usual Indo-European word is represented by Old English eoh, Greek hippos, Latin equus, from PIE root *ekwo-. Another Germanic "horse" word is Old English vicg, from Proto-Germanic *wegja- (source also of Old Frisian wegk-, Old Saxon wigg, Old Norse vigg), which is of uncertain origin. In many other languages, as in English, this root has been lost in favor of synonyms, probably via superstitious taboo on uttering the name of an animal so important in Indo-European religion. For the Romanic words (French cheval, Spanish caballo) see cavalier (n.); for Dutch paard, German Pferd, see palfrey; for Swedish häst, Danish hest see henchman. As plural Old English had collective singular horse as well as horses, in Middle English also sometimes horsen, but horses has been the usual plural since 17c.

Used at least since late 14c. of various devices or appliances which suggest a horse (as in sawhorse), typically in reference to being "that upon which something is mounted." For sense of "large, coarse," see horseradish. Slang use for "heroin" is attested by 1950. To ride a horse that was foaled of an acorn (1670s) was through early 19c. a way to say "be hanged from the gallows." Horse latitudes first attested 1777, the name of unknown origin, despite much speculation. Horse-pistol, "large one-handed pistol used by horseback riders," is by 1704. A dead horse as a figure for something that has ceased to be useful is from 1630s; to flog a dead horse "attempt to revive interest in a worn-out topic" is from 1864.

HORSEGODMOTHER, a large masculine wench; one whom it is difficult to rank among the purest and gentlest portion of the community. [John Trotter Brockett, "A Glossary of North Country Words," 1829]

The term itself is attested from 1560s. The horse's mouth as a source of reliable information is from 1921, perhaps originally of racetrack tips, 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 from the American Civil War and appears to have been originally one of Abe Lincoln's stories. Horse-and-buggy meaning "old-fashioned" is recorded from 1926 slang, originally in reference to a "young lady out of date, with long hair." To hold (one's) horses "restrain one's enthusiasm, be patient" is from 1842, American English; the notion is of keeping a tight grip on the reins.

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horse sense (n.)
1832, American English colloquial, from horse (n.), perhaps in referfence to the animal's qualities, or to the abilities of hostlers and coachmen with the animals, perhaps from the same association of "strong, large, coarse" found in horseradish.
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pack-horse (n.)

"horse used in carrying burdens," c. 1500, from pack (n.) + horse (n.).

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horse-meat (n.)
c. 1400, "food for horses," from horse (n.) + meat (n.). From 1853 as "horse-flesh."
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high horse (n.)
originally (late 14c.) "fine, tall horse; war horse, charger" (high steed is from c. 1300), also, like high hall, used in the sense "status symbol;" figurative sense of "airs, easily wounded dignity" in mount (one's) high horse "affect airs of superiority" is from 1782 (Addison has to ride the great horse in the same sense, 1716). Compare French monter sur ses grands chevaux. "The simile is common to most languages" [Farmer].
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horse-whip (n.)
also horsewhip, 1690s, from horse (n.) + whip (n.). As a verb, "to flog with a horse-whip," from 1768. Related: Horse-whipped; horse-whipping.
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horse-flesh (n.)
also horseflesh, c. 1400, "horses collectively;" 1530s, "meat from a horse," from horse (n.) + flesh (n.). From 1520s as a color-name.
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cart-horse (n.)
"horse that draws a cart," late 14c., from cart (n.) + horse (n.).
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