A fanny thief is the male version of this. It is when a man intends to steal a woman away from his buddy who wanted to hook up with her first.
Stealing is thieving, and stealing this woman away would be for the intention of fanny. Hence the appropriate name 'fanny thief'.
Spencer Matthews: "mate, i just want pancakes"
Jamie Laing: "no pancakes for you fanny thief!"