1. A crust discharged from and covering a healing wound.
2. Scabies or mange in domestic animals or livestock, especially sheep.
3. 1. Any of various plant diseases caused by fungi or bacteria and resulting in crustlike spots on fruit, leaves, or roots.
   2. The spots caused by such a disease.
4. Slang. A person regarded as contemptible.
5. 1. A worker who refuses membership in a labor union.
   2. An employee who works while others are on strike; a strikebreaker.
   3. A person hired to replace a striking worker.

1. To become covered with scabs or a scab.
2. To work or take a job as a scab.

[Middle English, from Old Norse skabb.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.