Brewer's: Double up
(To). To fold together. “To double up the fist” is to fold the fingers together so as to make the hand into a fist.
I doubled him up. I struck him in the wind, so as to make him double up with pain, or so as to leave him “all of a heap.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894