muscle

Where it comes from

Muscle ordinarily means body tissue that produces movement, or physical strength. English muscle comes through Middle English from Latin musculus. Latin musculus is the diminutive of mus, mouse, so it literally means little mouse; the Latin word itself was used for a bodily muscle. The traditional explanation compares the shape or movement of a flexing muscle under skin to a mouse.

How the meaning developed

Greek mys likewise meant both mouse and muscle, showing that the animal-to-anatomy association was not confined to the English borrowing chain.