A "scab" a derogatory term used to describe a strike breaker. The term is actually an old English insult, and has been in use to describe a despicable person since at least 1590. In the 1700s, someone who refused to join a labor union was called a scab, and by 1806, the word had reached its modern usage. More temperate labor activists and unions use the term “strike breaker” to refer to a scab, but the word is often used in speeches and literature which are designed to fire up the strikers.

Whenever workers refuse to work in order to gain concessions, it is called a strike. Strikes were an important part of the early labor movement, which agitated for safer working conditions, better pay, and more reasonable hours. These early strikes were often brutally put down, and workers had a choice between going back to work and starving. Labor unions attempted to help with this by organizing workers who paid dues which could be used to support them during a strike. A single scab could greatly weaken the cause of the union.

In response to more organized labor, companies started to recruit people who were willing to break the strike. These people might be existing employees or outside contractors. By crossing the picket line of strikers marching and holding signs for better working conditions, the scab hurts the cause of the workers. For this reason, the term “scab” started to become widespread, as a scab was someone who behaved dishonorably in 18th century culture, and retaliation against scabs could sometimes be brutal.

"Scab" is also used to refer to workers who cave too easily to concessions offered by a company. Labor activists believe that striking is an effective tool, and that if the workers band together, they can achieve their goals. Workers who agree to partial concessions weaken the cause of the whole, as do people who work through the strike. Sometimes, striking workers are surprised when the temporary workers hired to replace them up becoming permanent.

When a strike is in progress, people who support it should refrain from crossing the picket line. Workers typically form a band in front of the company they work for to inform people that a strike is going on, and why. By crossing the picket line, scabs and consumers indicate that they are not concerned about the rights of the workers, and they weaken the case of the strikers. In some cases, people may feel forced to cross a picket line. For example, university students may be told that they must attend class whether or not university staff are striking. This tactic is often used to cause strikes to fail.