A “wolf’s head” is someone who has done something so terrible that he or she is not considered human and anyone can and should kill him.
Crispin (Asta's Son) saw something he shouldn't have in the woods. He got in a fight subsequently after seeing Aycliffe, and as a result he hid in the woods, only to return to find his house being torn down.
The steward, John Aycliffe, accuses Crispin of theft, saying he stole from the Manor House. The priest informs Crispin that he has been declared a “wolf’s head” by the steward. He asks him if he knows what that means. Crispin does.
“That…I’m not considered human,” I said, my voice faltering. “That anyone may … kill me. Is that why they pulled down our house?” (p. 36)
Crispin did not even know his name until the priest gave him the Cross of Lead and told him his name.