late 15c., "reciprocally given and received," originally of feelings, from Old French mutuel (14c.), from Latin mutuus "reciprocal, done in exchange," from PIE root *mei- (1) "to change, go, move," "with derivatives referring to the exchange of goods and services within a society as regulated by custom or law" [Watkins].

The meaning "common" is from 1630s. "Used in this sense loosely and improperly (but not infrequently, and by many writers of high rank), especially in the phrase a mutual friend" [Century Dictionary].

That is common which pertains equally to two or more persons or things. That is mutual which is freely interchanged: mutual love, affection, hatred. The word is sometimes incorrectly used for common: our mutual friend, a phrase of very frequent occurrence, no doubt owing to the perfectly correct 'mutual friendship.' [J.H.A. Günther, "English Synonyms Explained & Illustrated," Groningen, 1904]

Mutual Admiration Society (1851) seems to have been coined by Thoreau. Mutual fund is recorded from 1950. The Cold War's mutual assured destruction is attested from 1966. (Assured destruction was an early 1960s term in U.S. military policy circles in reference to nuclear weapons as a deterrent, popularized c. 1964 by Robert McNamara, U.S. Secretary of Defense under Lyndon Johnson, e.g. statement before House Armed Services Committee, Feb. 18, 1965; the mutual perhaps was first added by Donald Brennan, conservative defense analyst and a public critic of the policy, who also noted the acronym MAD.)

1845, in reference to the doctrine of French anarchist/socialist Pierre-Joseph Proudhon (1809-1865) that individual and collective well-being is attainable only by mutual dependence, from French mutuellisme. In biology, "a symbiosis in which two organisms living together mutually and permanently help and support one another," from 1876, from mutual + -ism.

"exhibiting or characteristic of mutualism," 1850, from mutualist "advocate of mutualism" (1848); see mutualism.

"reciprocity, interchange," 1580s, from mutual + -ity.

"reciprocally, in a manner of giving and receiving," 1530s, from mutual + -ly (2). The meaning "conjointly, in common" (1590s) is "Held to be an erroneous use" [Century Dictionary]. Mutually exclusive is recorded by 1650s.

also muu-muu, "loose-fitting dress, usually of bright colors and patterns," 1923, from Hawaiian mu'u mu'u, literally "cut off," name given to the local adaptation of the dresses given to island women by the wives of early 19c. Christian missionaries "in the early days when a few flowers sufficed for a garment" [Don Blanding, "Hula Moons," 1930]. So called because the native style hangs from the shoulder and omits the high neck and the train.

1935, proprietary name for piped music, supposedly a blend of music and Kodak, said to have been coined by Gen. George Squier (1865-1934), who, among his other important inventions, developed the system of background music for workplaces c. 1922.

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.