late 14c., mosel, "device put over an animal's mouth to stop it from biting, eating, or rooting," from Old French musel "muzzle," also "snout, nose" (12c., Modern French museau), from muse "muzzle," from Gallo-Roman *musa "snout" (source also of Provençal mus, Old Spanish mus, Italian muso), a word of unknown origin, possibly related to Latin morsus "bite" (but OED finds "serious difficulties" with this).
Meaning "projecting jaws and nose of the head of an animal" is from early 15c.; sense of "open end of a firearm" is recorded from 1560s. Muzzle-loader "gun loaded from the muzzle" (opposed to breech-loader) is by 1858.
"to put a muzzle on, bind or confine the mouth of to prevent from biting or eating," early 15c., moselen, from muzzle (n.). Figurative use, "to gag, silence," is from 1610s. Related: Muzzled; muzzling.
late 14c., "small bagpipe," from Old French musette "bagpipe" (13c.), from muser "to play the bagpipe, make music," from mus "mouth, muzzle," from Medieval Latin musum (see muzzle (n.)). By 1788 as "a composition for or as though for a musette," a quiet pastoral melody, usually in imitation of a bagpipe, from this sense in French.
MUSETTE. The name of an air generally written in common-time, and the character of which is always soft and sweet. From the style of this air, dances were formerly invented of a similar cast, and which were also called Musettes. Musette was also the name formerly given to a small kind of bagpipe much used in most countries of Europe, the performers on which were called Musars. [Thomas Busby, "A Complete Dictionary of Music," 1806]
"contraction of the preputial orifice," 1670s, from Greek phimosis, literally "muzzling," from phimos "a muzzle, a gag," a word of unexplained etymology.
"a person's facial expression," 1510s, probably a shortening of Middle English demean "bearing, demeanor" (see demeanor) and influenced by French mine "appearance, facial expression," which is of unknown origin, possibly Celtic (compare Breton min "beak, muzzle, nose," Irish men "mouth").