"Milking somebody's goat" Also means that you beat them at whatever the situation was.
It can also mean you literally are going to milk the goat someone else owns, but it is used less commonly in that way.
P1: "I'm going to milk your goat."
P2 (Intimidated, annoyed, and insulted): "W-well go ahead and tr-try it!"
Bob: "Woah, did you see that pingpong player destroy Roger?!"
Greg: "Yeah, he totally milked Roger's goat!"