"explosive cap," 1819, agent noun from prime (v.).

"hat, cap," 1846, British English, of unknown origin.

type of flat cap, 1828, from French barrette, cognate with Spanish birreta, Italian beretta (see biretta).

also berret, "round, flat, woolen cap," originally worn by Basque peasants, 1827 as a fashionable accessory, from French béret, 19c., from dialect of Béarn, from Old Gascon berret "cap," from Medieval Latin birretum, diminutive of Late Latin birrus "a large hooded cloak," a word perhaps of Gaulish origin. For the clerical version, see biretta.

also red-cap, "porter at a railroad station," 1914, American English, from red (adj.1) + cap (n.). Earlier it was the name of the goldfinch, a type of domestic hen, a long-toothed spectre in Scottish castles, and anyone who wears a red hat (1540s).

type of cap worn in Canada, 1871, from Canadian French variant of French toque (see toque).

1856, "put a cap again on" something, originally typically a cartridge, from re- "again" + cap (v.). The specific sense of "put a strip of rubber on the tread of a tire" is from 1920s; hence, as a noun, "a recapped auto tire" (1939). As a shortened form of recapitulate (v.), it dates from 1920s. Related: Recapped; recapping.

late 15c., "native of Phrygia," region in ancient Asia Minor. As an adjective, "of, originating in, or relating to Phrygia," by 1570s. The Phrygian mode in ancient Greek music theory was held to be "of a warlike character." The Phrygian cap (1796) was the type adopted by freed slaves in Roman times, and thus it was subsequently identified as the cap of Liberty.

early 14c., "close-fitting cap worn by women," from French cale "cap," back-formation from calotte, from Italian callotta, from Latin calautica "type of female headdress with pendent lappets," a foreign word of unknown origin.

The "cap" sense was the main one until 17c. Medical use, in reference to various membranes, dates to late 14c.; especially of the amnion enclosing the fetus before birth from 1540s. This, if a child was born draped in it, was supersititously supposed to betoken prosperity, give the gift of eloquence, and protect against drowning (18c. seamen paid dearly for one, and cauls were advertised for sale in British newspapers through World War I).

also mortar-board, 1823, "square board used by masons to hold mortar for plastering," from mortar (n.1) + board (n.1). By 1854 in reference to the academic cap, probably so called because it resembles the mason's board. Earlier it was called a mortar cap (1680s) or simply morter (c. 1600), from French mortier.