When a man slides his penis between his partner's buttocks cheeks, but does not penetrate. Similar to "titty-fucking," but instead involving the butt cheeks.
When you are at a music festival and the performance is displeasing.
Mike "Have you still got that shit bag"
Sally "No, I chucked it at Gorillaz"
Mike "Nice, was it a direct hit?"
Sally "Yeh it was a perfect shitbagging, exploded on impact!"