Scab
After God had finished the rattlesnake, the toad, and the vampire, He had some awful substance left with which He made a scab. A scab is a two-legged animal with a corkscrew soul, a waterlogged brain, and a combination backbone made of jelly and glue. Where others have hearts, he carries a tumor of rotten principles.
When a scab comes down the street, men turn their backs and angels weep in heaven, and the devil shuts the gates of hell to keep him out. No man has a right to scab as long as there is a pool of water deep enough to drown his body in, or a rope long enough to hang his carcass with. Judas Iscariot was a gentleman compared with a scab. For betraying his Master, he had character enough to hang himself. A scab hasn't.
Esau sold his birthright for a mess of pottage. Judas Iscariot sold his savior for thirty pieces of silver. Benedict Arnold sold his country for a promise of a commission in the British Army. The modern strikebreaker sells his birthright, his country, his wife, his children, and his fellow men for an unfulfilled promise from his employer, trust, or corporation.
— Attributed to Jack London
A strikebreaker, pejoratively known as a scab, is a person who works despite an ongoing strike action, undercutting the impact of the strike on profit rates and therefore its chance of success. Scabs are usually unorganized proletarians who were not employed by the company before the trade union dispute but hired after or during the strike to keep the organization running, but the term may also refer to existing workers (union members or not) who continue to work.
Scabbing during a strike is typically known as "crossing the picket line".