The Collaborative International Dictionary of English v.0.48:

Stoat \Stoat\, n. [OE. stot a stoat, horse, bullock; perhaps
   originally only of male animals, and akin to D. stooten to
   push, E. stutter; cf. Icel. st?tr a bull, Sw. stut a bullock.
   Cf. Stot.] (Zool.)
   The ermine in its summer pelage, when it is reddish brown,
   but with a black tip to the tail. The name is sometimes
   applied also to other brown weasels.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stoat
    n 1: the ermine in its brown summer coat with black-tipped tail