scab

1. An incrustation over a sore, wound, vesicle, or pustule, formed during healing. (countable, uncountable)
2. The mange, especially when it appears on sheep. (countable, uncountable)
3. Any of several different diseases of potatoes producing pits and other damage on their surface, caused by streptomyces bacteria (but formerly believed to be caused by a fungus). (uncountable)
4. Common scab, a relatively harmless variety of scab (potato disease) caused by Streptomyces scabies. (countable, uncountable)
5. Any one of various more or less destructive fungal diseases that attack cultivated plants, forming dark-colored crustlike spots. (countable, uncountable)
6. A slight irregular protuberance which defaces the surface of a casting, caused by the breaking away of a part of the mold. (countable, uncountable)
7. A mean, dirty, paltry fellow. (countable, uncountable)
8. A worker who acts against trade union policies; any picket crosser (strikebreaker), and especially one with devotion to union busting. (countable, slang, uncountable)

Plural scabs