Brewer's: Leach, Leachcraft
A leach is one skilled in medicine, and “leach-craft” is the profession of a medical man. (Anglo-Saxon, laece, one who relieves pain, leachcraft.)
And straight way sent, with carefull diligence. To fetch a leach the which had great insight In that disease.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894