How Well Do You Know English Slang?
early 15c., possibly from Anglo-French *dormouse "tending to be dormant" (from stem of dormir "to sleep," see dormer), with the second element mistaken for mouse; or perhaps it is from a Middle English dialectal compound of mouse and Middle French dormir. The rodent is inactive in winter. French dormeuse, fem. of dormeur "sleeper" is attested only from 17c.