muscle

From Latin musculus, literally “little mouse” — a flexing bicep was thought to look like a small mouse moving under the skin.

The nickname stuck so thoroughly that it became the standard anatomical term, while the image of a mouse under the skin faded out of how people actually think about the word.

Musculus is a diminutive of mus (mouse) — the same root behind mouse itself, and, much later, the computer mouse.