"the horizontal shelf or hood above a fireplace," 1680s, from mantel + piece (n.1).
Entries linking to mantelpiece
mantel n.
c. 1200, "short, loose, sleeveless cloak," variant of mantle (q.v.). Sense of "movable shelter for soldiers besieging a fort" is from 1520s.
The meaning "timber or stone supporting masonry above a fireplace" is attested by 1510s; it is a shortened form of Middle English mantiltre "mantle-tree" (late 15c.) "beam of oak or some other hard wood above a fireplace or oven" (with tree in the now-obsolete sense of "beam"). But the exact meaning of mantle in that had become obscure by the 19c.
In a fire-place, the mantle or mantlepiece, may have been either a covered or chimney-piece; or the part below it to which a hanging, for the sake of making a flue for the wind to draw up the fire, was attached. The details, however, of this are uncertain. [Robert Gordon Latham, "A Dictionary of the English Language," 1882]
Mantel-clock "clock intended to sit on a mantle-shelf," is by 1824.
piece n.1
c. 1200, pece, "fixed amount, measure, portion;" c. 1300, "fragment of an object, bit of a whole, slice of meat; separate fragment, section, or part," 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.
Meaning "separate article forming part of a or group" is from c. 1400; that of "specimen, instance, example" is from 1560s. Sense of "portable firearm" is from 1580s, earlier "artillery weapon" (1540s). The meaning "chessman" is from 1560s. Meaning "a period of time" is from early 14c.; that of "a portion of a distance" is from 1610s; that of "literary composition" dates from 1530s.
Piece of (one's) mind "one's opinion expressed bluntly" is from 1570s. Piece of work "remarkable person" echoes Hamlet. Piece as "a coin" is attested in English from c. 1400, hence piece of eight, old name for the Spanish dollar (c. 1600) of the value of 8 reals and bearing a numeral 8. Adverbial phrase in one piece "whole, undivided, without loss or injury" is by 1580s; of a piece "as of the same piece or whole" is from 1610s.