From The Concise Oxford Dictionary of current English:
scab
1.Dry rough incrustation formed over sore in healing; mange, itch, or similar skin-disease; fungous plant-disease causing scablike roughness.
2. (arch.) Mean low fellow.
3. Person who refuses to join strike or trade union or takes striker's place or breaks rules of trade union or group.
4. Hence ~bed.
5. (Of sore) form scab, heal over; act as scab.