The word muscle came to us in the late fourteenth century from the Middle French word muscle, with basically the same meaning (there were some weird alterations along the way, like muskylle and muscule, but history has canceled those out). This too was borrowed in the fourteenth century, albeit from Latin. In this case, the word in question was musculus, which too meant muscle... figuratively. It carried quite a different literal meaning, that of "little mouse"! The connection occurred because muscles really do sort of look like tiny murine lumps under your skin. Musculus is a diminutive of mus, the normal word for mouse. Through Proto-Italic mus, this is reconstructed as having derived from the Proto-Indo-European word muhs, still meaning "mouse". Curiously, correlations between mice and muscles can also be found in languages from Arabic to Greek to German, as well.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.