From WordNet (r) 2.1 (2005) (link)

  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]

From English Etymology Dictionary (link)

  polecat
  
  1320, first element is probably Anglo-Fr. pol, from O.Fr. poule "fowl,
  hen," so called because it preys on poultry. The other alternative is
  that the first element is from O.Fr. pulent "stinking," for obvious
  reasons. Originally the European Putorius foetidus; also applied to
  related U.S. skunks since 1688.
  
  

From Merriam-Webster's Collegiate Dictionary, 11th Edition (2003) (link)

  polecat noun (plural polecats or polecat)
   Etymology: Middle English polcat, probably from Middle
   French poul, pol cock + Middle English cat; probably from its
   preying on poultry — more at pullet Date: 14th century 1.
   any of several carnivorous mammals (as of the genera Mustela or
   Vormela) of the weasel family; especially  a brown to black
   European mammal (M. putorius) from which the domesticated ferret is
   derived 2.  skunk
  
  

From Oxford English Reference Dictionary (link)

polecat
n.
1 Brit. a small European brownish-black fetid flesh-eating mammal, Mustela putorius, of the weasel family.
2 US a skunk.
Etymology: pole (unexplained) + CAT

From Collins COBUILD Advanced Learner's English Dictionary (link)

  polecat
  (polecats)
    A polecat is a small, thin, fierce wild animal. Polecats have a very unpleasant smell.
   N-COUNT
  
  
  

From English Explanatory Dictionary (link)

  polecat
  ˈpəulkæt n. 1 Brit. a small European brownish-black fetid
  flesh-eating mammal, Mustela putorius, of the weasel family. 2 US a
  skunk. [pole (unexplained) + CAT]
  
  
  

From Webster's Revised Unabridged Dictionary (1913) (link)

  Polecat \Pole"cat`\, n. [Probably fr. F. poule hen, and
     originally, a poultry cat, because it feeds on poultry. See
     Poultry.] (Zo["o]l.)
     (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.

From Soule's Dictionary of English Synonyms (link)

  polecat
  n.
   1. [Of Europe.] Fitchew, fitch, fitchet, weasel, foumart, foulmart
   (Mustela putorius).
   2. [Of North America.] Mink, minx (Mustela vison).
  
  
  

From Moby Thesaurus II by Grady Ward, 1.0 (link)

  53 Moby Thesaurus words for "polecat":
     Cape polecat, animal, ape, bar, bear, beast, cavy, chimp,
     chimpanzee, coon, cur, dog, ferret, foumart, glutton, groundhog,
     guinea pig, hedgehog, hound, hyena, insect, mongrel, monk, monkey,
     mousehound, opossum, pig, porcupine, possum, prairie dog,
     quill pig, raccoon, reptile, rotten egg, serpent, skunk, snake,
     stink ball, stink bomb, stinkard, stinker, stinkpot, swine,
     varmint, vermin, viper, weasel, whelp, whistle-pig, wolverine,
     woodchuck, worm, zoril