When you see a girl you like and you hook up with her without getting a good look at her friend. You proceed to call your wingman over to hook up with her friend. Then you realize you hooked your friend up with the better looking girl. Hence, the reverse wingman.
*after the hook up*
Elliot: "Man I hooked up with a hot chick last night"
K-Dub: "So did I. I appreciate being the wingman with a the better looking chick. I'm always cool with being the reverse wingman."