An Etymological Dictionary of Astronomy and Astrophysics

1. A tissue composed of bundles of elongated cells capable of contraction and relaxation to produce movement in an organ or part.
   

   2. An organ composed of muscle tissue (TheFreeDictionary.com).

Etymology (EN): From M.Fr. muscle “muscle, sinew” and directly from L. musculus “a muscle,” literally “little mouse,” diminutive of mus “mouse;” → -ule. So called because the shape and movement of some muscles (notably biceps) were thought to resemble mice.

Etymology (PE): Mâhice, literally “small fish, fish form,” because of apparent resemblance of some muscles to fish, from mâhi, → fish, + -ce diminutive/similarity suffix; variant mâyce.