Kids who fall and skin their knees know what a scab is: it's the crusty patch of skin that forms when a scrape or cut is healing.
A wound will form a scab as it heals, whether it's a cut or a burn. Skin has an amazing ability to repair itself, and a scab is evidence of this. Scab is also a slang term for someone who crosses a picket line during a strike, choosing to work instead of joining coworkers in protesting low wages or harsh treatment by an employer. This kind of scab first meant "unpleasant person" in the late 1500s.