It is what a guy usually says when referring to what he wants to do to a "butter face" or a girl with a really hot body but an ugly face. The action is when you have sexual intercourse with a "butter face" you must put a bag over their head, so as to not see their face. Making the act less awkward.
Man I would totally bang her if she didn't have a face." -"Dude, Bag n' Tag.