From my understanding (mainly from Billy Elliott, TBH), a "scab" is a worker who continues working even when the union has called a strike.

It's normally a non union member who is hired to do the job striking union member won't do.

Ahh, that makes sense! Maybe keep the inflammatory language out of your original comment, though. A lot of people browsing these forums and may not be familiar with local slang terms/jargon like that, and your original comment sounded like a definition.