after a night of heavy drinking, eating, etc., the act of waking up the next morning and taking the most violent, earth-shattering dump, in which it sprays the walls of the crapper with the powerful force of a fire hose!!!
"oh man, i must have had like fifteen heinekens last night before we got those chili cheese dogs....uh-oh...i'll be right back...."
Mangina or Man-gina would be gay slang for anus, ass or man-cunt. Often used on the leather scene but on the whole it is a generic term for a guy's butt hole.
that guy gets so much action his man-gina is like a inner tube.