"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.
"automated cafeteria," 1903, probably from automatic (adj.), though the system itself is said to have originated in Germany, and the word may be from German. Earlier it meant "an automaton" (1670s).
"Maxim automatic gun," 1899, of imitative origin, soldiers' slang from the Boer War. For the ornamental tuft, see pompom.
"done by automatic equipment," 1952, American English, adjective based on automation.