The word 'muscle' literally means 'little mouse'.
Latin 'musculus' is the diminutive of 'mus', mouse. Roman physicians thought a flexing biceps under the skin looked like a mouse scurrying back and forth. The image was so apt that it travelled into many languages: English 'muscle', French 'muscle', German 'Muskel'. The Greek 'mys' likewise means both mouse and muscle.
Useful: To remember a technical term, look for the everyday image it grew out of.
Source: en.wikipedia.org