It's when you're fooling around and you get in such a position that her tits are actually on your ass. This is not great for easy access stimulation since it is a awkward position, but erotic nonetheless. It can also be used as an expression of grief, since it reminds one of better times.
Guy 1: Dude, we're gonna miss our boat.
Guy 2: Really? I thought we had plenty of time.
Guy 1: Yeah. This always happens.
Guy 2: Fuck... Aww, "tits on my ass!"