Scab  —n. 1 crust over a healing cut, sore, etc. 2 (often attrib.) Colloq. Derog. Blackleg. 3 skin disease, esp. In animals. 4 fungous plant disease. —v. (-bb-) 1 colloq. Derog. Act as a blackleg. 2 form a scab, heal over.  scabby adj. (-ier, -iest). [old norse: cf. *shabby]