mouthpiece (n.)

Old English muþ "oral opening of an animal or human; opening of anything, door, gate," from Proto-Germanic *muntha- (source also of Old Saxon, Old Frisian muth, Old Norse munnr, Danish mund, Middle Dutch mont, Dutch mond, Old High German mund, German Mund, Gothic munþs "mouth"), with characteristic loss of nasal consonant in Old English (compare tooth), probably an IE word, but the exact etymology is disputed. Perhaps from the source of Latin mentum "chin" (from PIE root *men- (2) "to project," on the notion of "projecting body part"), presuming a semantic shift from "chin" to "mouth."

In the sense of "outfall of a river" it is attested from late Old English; as the opening of anything with capacity (a bottle, cave, etc.) it is recorded from mid-13c. Mouth-organ attested from 1660s. Mouth-breather is by 1883. Mouth-to-mouth "involving contact of one person's mouth with another's" is from 1909.

Word of mouth "spoken words, oral communication" (as distinguished from written words) is by 1550s. To put words in (someone's) mouth "represent as having said what one did not say" is from late 14c.; to take the words out of (someone's) mouth "anticipate what one is about to say" is from 1520s. To be down in the mouth "dejected" (1640s) is from the notion of having the corners of the mouth turned downward.

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 class 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.