The Collaborative International Dictionary of English v.0.48:

Cur \Cur\ (k[^u]r), n. [OE. curre, kur; cf. dial. Sw. kurre dog,
   OD. korre watchdog, and Icel. kurra to murmur, grumble, Sw.
   kurra to rumble, croak, Dan. kurre to coo, whirr; prob. of
   imitative origin.]
   1. A mongrel or inferior dog.
      [1913 Webster]

            They . . . like to village curs,
            Bark when their fellows do.           -- Shak.
      [1913 Webster]

   2. A worthless, snarling fellow; -- used in contempt.
      [1913 Webster]

            What would you have, you curs,
            That like nor peace nor war?          --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cur
    n 1: an inferior dog or one of mixed breed [syn: cur,
         mongrel, mutt]
    2: a cowardly and despicable person

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "cur":
   animal, beast, bounder, dog, hound, hyena, insect, lurcher,
   mongrel, mutt, pariah dog, pig, polecat, reptile, riffraff, rotter,
   scum, serpent, shit, shithead, skunk, snake, stinkard, stinker,
   swine, toad, varmint, vermin, viper, whelp, worm