c. 1200, "fixed amount, measure, portion," from Old French piece "piece, bit portion; item; coin" (12c.), from Vulgar Latin *pettia, probably from Gaulish *pettsi (compare Welsh peth "thing," Breton pez "piece, a little"), perhaps from an Old Celtic base *kwezd-i-, from PIE root *kwezd- "a part, piece" (source also of Russian chast' "part"). Related: Pieces.
Sense of "portable firearm" first recorded 1580s; that of "chessman" is from 1560s. Meaning "person regarded as a sex object" is first recorded 1785 (compare piece of ass under ass (n.2); human beings colloquially have been piece of flesh from 1590s; also compare Latin scortum "bimbo, anyone available for a price," literally "skin"). Meaning "a portion of a distance" is from 1610s; that of "literary composition" dates from 1530s. Piece of (one's) mind is from 1570s. Piece of work "remarkable person" echoes Hamlet. Piece as "a coin" is attested in English from 1570s, hence Piece of eight, old name for the Spanish dollar (c. 1600) of the value of 8 reals.
PIECE. A wench. A damned good or bad piece; a girl who is more or less active and skilful in the amorous congress. Hence the (Cambridge) toast, may we never have a PIECE (peace) that will injure the constitution. ["Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence," London, 1811]