The Collaborative International Dictionary of English v.0.48:

Bully \Bul"ly\ (b[.u]l"l[y^]), n., Bully beef \Bul"ly beef`\
   (b[.u]l"l[y^] b[=e]f`). [F. bouilli boiled meat, fr. bouillir
   to boil. See Boil, v. The word bouilli was formerly
   commonly used on the labels of canned beef.]
   Pickled or canned beef.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

bully beef
    n 1: beef cured or pickled in brine [syn: bully beef, corned
         beef, corn beef]