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.
      [1913 Webster]

   2. The itch in man; also, the scurvy. [Colloq. or Obs.]
      [1913 Webster]

   3. The mange, esp. when it appears on sheep. --Chaucer.
      [1913 Webster]

   4. A disease of potatoes producing pits in their surface,
      caused by a minute fungus (Tiburcinia Scabies).
      [1913 Webster]

   5. (Founding) A slight irregular protuberance which defaces
      the surface of a casting, caused by the breaking away of a
      part of the mold.
      [1913 Webster]

   6. A mean, dirty, paltry fellow. [Low] --Shak.
      [1913 Webster]

   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]
      [1913 Webster]

   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.]

The Collaborative International Dictionary of English v.0.48:

Scab \Scab\, v. i. [imp. & p. p. Scabbed; p. pr. & vb. n.
   Scabbing.]
   1. To become covered with a scab; as, the wound scabbed over.
      [1913 Webster]

   2. to take the place of a striking worker.
      [PJC]

WordNet (r) 3.0 (2006):

scab
    n 1: someone who works (or provides workers) during a strike
         [syn: scab, strikebreaker, blackleg, rat]
    2: the crustlike surface of a healing skin lesion
    v 1: form a scab; "the wounds will eventually scab"
    2: take the place of work of someone on strike [syn: fink,
       scab, rat, blackleg]