Scab (noun)

1. A rough crust that forms over a wound or sore to protect it while it heals.
2. A worker who refuses to join a trade union or to take part in a strike and who is willing to work in place of a striking worker.

Origin:

Old english sceabb, sceab, of germanic origin; related to dutch schurft and german schorf.

Examples:

1. He picked at the scab on his knee.
2. The scab eventually fell off, revealing new skin.
3. The workers accused him of being a scab.
4. The scab workers were not allowed in the union hall.
5. The scab caused the wound to bleed again.