Your biceps is named after a mouse.
The English word comes from Latin musculus, which means "a little mouse." It is the diminutive of mus, "mouse." Both Etymonline and Merriam-Webster give this as the source. Merriam-Webster adds that the Romans saw a resemblance between mice and the major muscles of the arms and legs.
The idea is easy to picture. When a muscle tenses, it bunches up and slides under the skin, something like a small animal moving beneath a blanket. Etymonline names the biceps as one muscle thought to resemble a mouse.
Greek has the same link. The Greek word mys means both "mouse" and "muscle," and its combining form gives the medical prefix myo-. Etymonline adds that other languages draw a similar link. German Maus covers both "mouse" and "muscle," Arabic has 'adalah "muscle" beside 'adal "field mouse," and Cornish calls the calf of the leg a "mouse of the leg." It is a repeated metaphor, not a one-off.
English took the word in the late 1300s for "contractible animal tissue consisting of bundles of fibers." Middle English also used lacerte, from the Latin for "lizard," for a muscle, which shows that small animals were the usual picture.
Then the sense stretched. "Strength, brawn" dates to 1850. "Force, violence" is recorded in American English from 1930. A "muscle car" is from 1969.
The verb came later. "To accomplish by strength" is recorded in 1913 (Etymonline), and Merriam-Webster dates its verb sense to about 1819, a discrepancy between the two. By 1929 in U.S. underworld slang, to "muscle" someone meant to coerce them by violence or pressure.
The Latin word thus carried a joke about size. A muscle was a mouse, but a mouse with a job to do.
Anatomy keeps the old image alive. Every time you flex, you are, by Roman reckoning, running a small mouse up your arm.