late 14c., from Middle French muscle "muscle, sinew" (14c.) and directly from Latin musculus "a muscle," literally "little mouse," diminutive of mus "mouse" (see mouse (n.)).
So called because the shape and movement of some muscles (notably biceps) were thought to resemble mice. The analogy was made in Greek, too, where mys is both "mouse" and "muscle," and its comb. form gives the medical prefix myo-. Cf. also Old Church Slavonic mysi "mouse," mysica "arm;" German Maus "mouse; muscle," Arabic 'adalah "muscle," 'adal "field mouse." In Middle English, lacerte, from the Latin word for "lizard," also was used as a word for a muscle.
Hence muscular and mousy are relatives, and a Middle English word for "muscular" was lacertous, "lizardy." Figurative sense of "force, violence, threat of violence" is 1930, American English. Muscle car "hot rod" is from 1969.