By drawing the blood of a witch you deprive her of her power of sorcery. Glanvil says that when Jane Brooks, the demon of Tedworth, bewitched a boy, his father scratched her face and drew blood, whereupon the boy instantly exclaimed that he was well.
“Blood will I draw on thee; thou art a witch.”
Dr. Sprenger computes that as many as nine millions of persons have suffered death for witchcraft since the bull of Innocent. (Life of Mohammed.) As late as 1705 two women were executed at Northampton for witchcraft.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894