Drawing the Nail
i.e. absolving oneself of a vow. In Cheshire, two or more persons would agree to do something, or to abstain from something, say drinking beer; and they would go into a wood, and register their vow by driving a nail into a tree, swearing to keep their vow as long as that nail remained in the tree. If they repented of their vow, some or all of the party went and drew out the nail, whereupon the vow was cancelled.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894