The Collaborative International Dictionary of English v.0.48:

Scab \Scab\ (sk[a^]b), n. [OE. scab, scabbe, shabbe; cf. AS.
   scaeb, sceabb, scebb, Dan. & Sw. skab, and also L. scabies,
   fr. scabere to scratch, akin to E. shave. See Shave, and
   cf. Shab, Shabby.]
   1. An incrustation over a sore, wound, vesicle, or pustule,
      formed by the drying up of the discharge from the diseased
      part.
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   2. The itch in man; also, the scurvy. [Colloq. or Obs.]
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   3. The mange, esp. when it appears on sheep. --Chaucer.
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   4. A disease of potatoes producing pits in their surface,
      caused by a minute fungus (Tiburcinia Scabies).
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   5. (Founding) A slight irregular protuberance which defaces
      the surface of a casting, caused by the breaking away of a
      part of the mold.
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   6. A mean, dirty, paltry fellow. [Low] --Shak.
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   7. A nickname for a workman who engages for lower wages than
      are fixed by the trades unions; also, for one who takes
      the place of a workman on a strike. [Cant]
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   8. (Bot.) Any one of various more or less destructive fungus
      diseases attacking cultivated plants, and usually forming
      dark-colored crustlike spots.
      [Webster 1913 Suppl.]