The Collaborative International Dictionary of English v.0.48:

Mongrel \Mon"grel\, n. [Prob. shortened fr. mongrel, and akin to
   AS. mengan to mix, and E. mingle. See Mingle.]
   The progeny resulting from a cross between two breeds, as of
   domestic animals; anything of mixed breed. --Drayton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mongrel \Mon"grel\, a.
   1. (Zool.) Not of a pure breed.
      [1913 Webster]

   2. Of mixed kinds; as, mongrel language.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mongrel
    n 1: derogatory term for a variation that is not genuine;
         something irregular or inferior or of dubious origin; "the
         architecture was a kind of bastard suggesting Gothic but
         not true Gothic" [syn: bastard, mongrel]
    2: an inferior dog or one of mixed breed [syn: cur, mongrel,
       mutt]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "mongrel":
   Cape Colored, Eurasian, animal, bastard, beast, cattalo, citrange,
   cross, crossbred, crossbreed, crossed, cur, dog, griffe,
   half blood, half-bred, half-breed, half-caste, high yellow, hinny,
   hound, hybrid, hyena, insect, interbred, ladino, liger, lurcher,
   mestee, mestiza, mestizo, metis, metisse, mixblood, mixed-blood,
   mulatto, mule, mustee, mutt, octoroon, pariah dog, pig, plumcot,
   polecat, quadroon, quintroon, reptile, sambo, serpent, skunk,
   snake, swine, tangelo, tigon, varmint, vermin, viper, whelp, worm,
   zebrass, zebrule