Meaning

noun one of the contractile organs of the body

noun animal tissue consisting predominantly of contractile cells

noun a bully employed as a thug or bodyguard

originFrom Middle English muscle, muscule, muskylle, and in part from Middle French muscle, from Latin mūsculus (“a muscle”, literally “little mouse”) because of the mouselike appearance of some muscles, from mūs (“mouse”). Doublet of mussel. More at mouse.

Etymology from Wiktionary, CC BY-SA 3.0