The Collaborative International Dictionary of English v.0.48:

Dormouse \Dor"mouse\, n.; pl. Dormice. [Perh. fr. F. dormir to
   sleep (Prov. E. dorm to doze) + E. mouse; or perh. changed
   fr. F. dormeuse, fem., a sleeper, though not found in the
   sense of a dormouse.] (Zool.)
   A small European rodent of the genus Myoxus, of several
   species. They live in trees and feed on nuts, acorns, etc.;
   -- so called because they are usually torpid in winter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dormouse
    n 1: small furry-tailed squirrel-like Old World rodent that
         becomes torpid in cold weather