scab (n.) Look up scab at Dictionary.com
mid-13c., "skin disease," developed from Old English sceabb "scab, itch" (related to scafan "to shave, scrape, scratch") and from Old Norse skabb "scab, itch," both from Proto-Germanic *skab- "scratch, shave," from PIE *(s)kep- "to cut, scrape, hack" (see scabies). Sense reinforced by cognate Latin scabies "scab, itch, mange" (from scabere "to scratch").

Meaning "crust which forms over a wound or sore" is first attested c.1400. Meaning "strikebreaker" first recorded 1806, from earlier sense of "person who refuses to join a trade union" (1777), probably from meaning "despicable person" (1580s), possibly borrowed in this sense from Middle Dutch.