When getting a blowjob, the girl deepthroats so far that she leaves her lipstick on your abdomen. Such a feat is usually a bragging point for the guy, hence why he may choose to keep it as long as possible, hence the "tattoo."
Guy 1: Dude, wtf is that?
Guy 2: Oh, that? That's my lipstick tattoo! The chick went so deep I may have punctured a lung.
Guy 1: Awesome!