When a guy takes it upon himself to introduce an unsuspecting woman to anal sex. Much dissapproval on the woman's behalf usually arupts upon the initial insertion into the rectum.
The best way to go about it is not to ask her... just try the good ol' poop shoot riot.
It's when you don't wipe your bum good enough after taking a poop. Your bum can get poo crusties and make your asshole itch.
I've got a bad case of butt rust this morning after all that drinking and pooping I did last night. Poop.