'musculus' originates from Latin, specifically the word 'musculus', where 'mus' meant 'mouse' and the diminutive suffix '-culus' meant 'little', hence 'little mouse'.

'musculus' passed into Old French as 'muscle' and then into Middle English as 'muscle', eventually yielding the modern English word 'muscle' (while the Latin form 'musculus' remained in scholarly and anatomical Latin).

Initially it literally meant 'little mouse'; over time it came to be used metaphorically for the bodily 'muscle' (because a contracting muscle was thought to look like a small moving mouse), which is its primary sense in modern anatomy.