noun
also mus·cle ˈməsəl
( -s )
Usage: often attributive
Etymology: Middle English muscle, from Old English muscelle, muscle, musle; akin to Old Saxon & Old High German muscula mussel, Middle Dutch mosschele; all from a prehistoric West Germanic word borrowed from (assumed) Vulgar Latin muscula, alteration of Latin musculus small mouse, muscle, mussel — more at muscle
1. : a marine bivalve mollusk of Mytilus or a related genus usually having an oval or elongated shell with a dark horny periostracum and being attached to the substrate by a byssus of fine threads secreted by the animal
2. : a freshwater bivalve mollusk of Unio, Anodonta, or related genera that is especially abundant in rivers of the central United States and has a shell with a lustrous nacreous lining much used in making buttons — called also freshwater clam, freshwater mussel