The Collaborative International Dictionary of English v.0.48:

Mutton \Mut"ton\, n. [OE. motoun, OF. moton, molton, a sheep,
   wether, F. mouton, LL. multo, by transposition of l fr. L.
   mutilus mutilated. See Mutilate.]
   1. A sheep. [Obs.] --Chapman.
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            Not so much ground as will feed a mutton. --Sir H.
                                                  Sidney.
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            Muttons, beeves, and porkers are good old words for
            the living quadrupeds.                --Hallam.
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   2. The flesh of a sheep.
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            The fat of roasted mutton or beef.    --Swift.
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   3. A loose woman; a prostitute. [Obs.]
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   Mutton bird (Zool.), the Australian short-tailed petrel
      (Nectris brevicaudus).

   Mutton chop, a rib of mutton for broiling, with the end of
      the bone at the smaller part chopped off.

   Mutton fish (Zool.), the American eelpout. See Eelpout.


   Mutton fist, a big brawny fist or hand. [Colloq.] --Dryden.

   Mutton monger, a pimp. [Low & Obs.] --Chapman.

   To return to one's muttons. [A translation of a phrase from
      a farce by De Brueys, revenons [`a] nos moutons let us
      return to our sheep.] To return to one's topic, subject of
      discussion, etc. [Humorous]
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            I willingly return to my muttons.     --H. R.
                                                  Haweis.
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WordNet (r) 3.0 (2006):

mutton
    n 1: meat from a mature domestic sheep [syn: mouton, mutton]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "mutton":
   agneau, bellwether, breast of lamb, ewe, ewe lamb, gigot,
   jambe de mouton, jumbuck, lamb, lambkin, leg of lamb,
   leg of mutton, mouton, ram, saddle of mutton, sheep, teg, tup,
   wether, yeanling