scab:
(plural: ‘scabs‘; for the right pronunciation of this word, please click here)
(an insulting remark for…) a person/worker/labourer who doesn’t participate in a labour strike, but helps the management of the factory or company by supplying men secretly to run the business that will weakens and break the other striking labourers’ efforts;
a strikebreaker; a blackleg,
A scab who crosses the picket line to earn a couple of extra bucks when others are on strike is looked down upon not only by their co-workers but also by the general public.
It has become a common practice for some business managements to hire scabs or strikebreakers in advance so that they can continue their business in case othere workers decide to go on a strike.
The labour unions have strict rules that every member must obey and when some workers want to be scabs, go on working without joining the strike, they are not allowed to be members any more and demand the managements to lay those scabs off once the disputes are sorted out.

