when you masturbate or have sex sitting down, the increased bloodflow to the pelvic region causes the your ass and the nether region to sweat, makeing your underwear (if your wearing any) very uncomfortable and wet.
nether region = area between a man's scrotum and ass hole. especially in the little nook right behind his balls.
the 'swamp' part of the word refers to "swamp ass" (similar thing but from running around)
Mike was fucking this girl, but then when he was done, the bedsheets keps sticking to his ass cuz he had sex swamp.
person1: guess what
person1: I was just in a circle jerk, and ..
person2: eww gross dude, stop there, circle jerk is nasty.
person1: .. and I went to stand up, and i had the worst wedgie cuz my boxers were sticking to my ass cheeks and i couldnt rearange them right.
person2: .. i think im gonna be sick.