Basically, this term means that you can't get something any cleaner (or purer) than it is. It's also meant as a compliment by saying that somebody's mother is not a skanky whore or streetwalker. This term appeared in the young adult edition of the novel adaptation of the 1979 sci-fi thriller"Alien".
Georgethe janitor tidied up the restroom so well it's not simply spic and span, it's as clean as your mother's reputation.
Ruler of Lake Titicaca. Rumored to have a bunghole that gets very angry if it does not receive toilet paper. Cornholio the Great is often seen walking around with his shirt over his head and his hands in the air, chanting songs about his power, and his bunghole.
"I am Cornholio! You do not want to face the wrath of my bunghole, for I need TP!"
Butthead: Shut up, Beavis! (uh huh huh huh)
Beavis: Um, okay. (heh heh heh heh).