black•leg (-leg´, -lāg´) n.
 1 an acute, usually fatal, infectious disease of young cattle and sheep, caused by a bacterium (esp. Clostridium chauvoei) and characterized by swelling and blackening of infected muscle tissue
 2 a disease of beets and related plants caused by a fungus (Phoma betae)
 3 a disease of potatoes caused by a bacterium (Erwinia atroseptica)
 4 [Informal] a gambler who cheats
 5 [Brit.] a strikebreaker