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]