mussel

From Middle English muscle, from Old English muscle, from Proto-West Germanic *muskulā, from Late Latin muscula, from Latin mūsculus, meaning "little mouse."

The Latin word mūsculus described something small and mouse-like, and that same root traveled through Late Latin and West Germanic into Old and Middle English before settling as the name for the shellfish.

It's a doublet of muscle — both words descend from the same Latin mūsculus, "little mouse," a fitting image since a flexing muscle was once thought to resemble a mouse moving under the skin.