welsh on someone
to renege on a bet or an agreement made with someone. (Also spelled welch.) You had better not welsh on me if you know what is good for you. Max welshed on the mob boss and made a lot of trouble for himself.
welsh on something (with someone)
to renege on a bet or agreement made with someone. (Also spelled welch.) Max welshed on his bet with Lefty. That was not a wise thing to do. It is not wise to welsh on a bet.