The Collaborative International Dictionary of English v.0.48:

Polecat \Pole"cat`\, n. [Probably fr. F. poule hen, and
   originally, a poultry cat, because it feeds on poultry. See
   Poultry.] (Zool.)
      (a) A small European carnivore of the Weasel family
          (Putorius f[oe]tidus). Its scent glands secrete a
          substance of an exceedingly disagreeable odor. Called
          also fitchet, foulmart, and European ferret.
      (b) The zorilla. The name is also applied to other allied
          species.
          [1913 Webster]

WordNet (r) 3.0 (2006):

polecat
    n 1: American musteline mammal typically ejecting an intensely
         malodorous fluid when startled; in some classifications put
         in a separate subfamily Mephitinae [syn: skunk,
         polecat, wood pussy]
    2: dark brown mustelid of woodlands of Eurasia that gives off an
       unpleasant odor when threatened [syn: polecat, fitch,
       foulmart, foumart, Mustela putorius]