fix (someone's) wagon
To take retaliatory action against someone. Primarily heard in US. After what he's done to me, I definitely want to fix his wagon—and this nasty rumor I've started is just the beginning.
fix someone's wagon
Fig. to punish someone; to get even with someone; to plot against someone. If you ever do that again, I'll fix your wagon! Tommy! You clean up your room this instant, or I'll fix your wagon! He reported me to the boss, but I fixed his wagon. I knocked his lunch on the floor.
fix (someone's) wagon
To get revenge on another.