Scab
Middle English (as a noun): from Old Norse skabb ; related to dialect shab (compare with shabby). The sense ‘contemptible person’ (dating from the late 16th century) was probably influenced by Middle Dutch schabbe ‘promiscuous woman’.
wiktionary
From Middle English scabb, scabbe (also as shabbe, schabbe > English shab), from Old English sċeabb and Old Norse skabb, both from Proto-Germanic *skabbaz(“scab, scabies”), from Proto-Indo-European *skabʰ-(“to cut, split, carve, shape”). Cognate with German Schabe(“scabies”), Danish skab(“scab, scabies”), Swedish skabb(“scab, scabies”), Latin scabies(“scab, itch, mange”). Related also to Old English scafan(“to scrape, shave”), Latin scabere(“to scratch”), English shabby.
etymonline
scab (n.)
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.
