"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.
also pistoleer, "one who uses a pistol, soldier armed with a pistol," 1570s from obsolete French pistolier, from pistole (see pistol).
1590s, former Spanish gold coin (not so called in Spanish), from French pistole, from Italian piastola, diminutive of piastra "plate or leaf of metal" (see piaster) and said to be so called for being smaller and thinner than the Crown. Compare earlier pistolet (1550s) "foreign coin," which OED says is from French pistolet "short firearm" (see pistol).
"sound of something moving rapidly," 1875, imitative. Zip gun "homemade pistol" is attested by 1950.
"a letter, a communication," Old English pistol, a shortening of epistol, from Latin epistola (see epistle). Compare postle from apostle.