muscle

Muscle is a noun which denotes the fibrous tissue that helps a body to move. Its language of origin is Latin: it comes from musculus, a noun meaning both "muscle" and "little mouse" since mus = mouse and -culus is a diminutive. If you flex your arm, you can see how the muscle there could be considered a little mouse. On its route from Latin to Middle English muscle traveled through Old French.

Sources: AHD & The New College Latin and English Dictionary by John Traupman