\Scab\, n. (Bot.)
Any one of various more or less destructive fungus diseases
attacking cultivated plants, and usually forming dark-colored
crustlike spots.

\Scab\ (sk[a^]b), n. [OE. scab, scabbe, shabbe; cf. AS.
sc[ae]b, 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.
2. The itch in man; also, the scurvy. [Colloq. or Obs.]
3. The mange, esp. when it appears on sheep. --Chaucer.
4. A disease of potatoes producing pits in their surface,
   caused by a minute fungus ({Tiburcinia Scabies}).
5. (Founding) A slight irregular protuberance which defaces
   the surface of a casting, caused by the breaking away of a
   part of the mold.
6. A mean, dirty, paltry fellow. [Low] --Shak.
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]

\Scab\, v. i. [imp. & p. p. {Scabbed}; p. pr. & vb. n.
{Scabbing}.]
To become covered with a scab; as, the wound scabbed over.