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]