BLACKLEG, a slang term in use since the i8th century for a cheat or turf swindler. Its modem use is as a term of reproach for a worker who refuses to come out on strike with his fellows, or, sometimes, but less correctly, for those who decline to join their appropriate trade society or union. Similarly, to "blackleg" is to return to work before the union or the general body of strikers have agreed to do so ; or to take the place of workers who are on strike or have been locked out.
Like the equally opprobrious word "scab," "blackleg" originally means a disease of cattle or sheep.