The Collaborative International Dictionary of English v.0.48:

Blackleg \Black"leg`\, n.
   1. A notorious gambler. [Colloq.]
      [1913 Webster]

   2. A disease among calves and sheep, characterized by a
      settling of gelatinous matter in the legs, and sometimes
      in the neck. [Eng.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

blackleg
    n 1: someone who works (or provides workers) during a strike
         [syn: scab, strikebreaker, blackleg, rat]
    v 1: take the place of work of someone on strike [syn: fink,
         scab, rat, blackleg]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "blackleg":
   Texas fever, anthrax, aphthous fever, bighead, black quarter,
   blackwater, blind staggers, bloody flux, broken wind,
   cattle plague, charbon, cheat, cheater, chiseler, cozener, crook,
   defrauder, diddler, distemper, fink, flimflam man, flimflammer,
   foot-and-mouth disease, gapes, glanders, gyp artist, gypper,
   heaves, hog cholera, hoof-and-mouth disease, hydrophobia, juggler,
   liver rot, loco, loco disease, locoism, mad staggers,
   malignant catarrh, malignant catarrhal fever, malignant pustule,
   mange, megrims, milzbrand, paratuberculosis, pip,
   pseudotuberculosis, quarter evil, rabies, rat, rinderpest, rot,
   scab, scabies, sheep rot, splenic fever, staggers, strikebreaker,
   stringhalt, swindler, swine dysentery, two-timer