blackleg

1. a person who continues working when fellow workers are on strike; a strike-breaker.

   the blacklegs who had arrived by rail were harassed by crowds of workers

   blackleg labour

2. an acute infectious bacterial disease of cattle and sheep, causing necrosis in one or more legs.
3. any of a number of plant diseases in which part of the stem blackens and decays.

verb

continue working when one's fellow workers are on strike.

early 18th century (in the sense ‘infectious disease of cattle and sheep’): from black + leg. The sense ‘strike-breaker’ dates from the mid 19th century; it may derive from an earlier sense ‘swindler, cheat’, with possible comparison to a crow or rook, black birds regarded as having thieving habits.