The ass bong roughly refers to the anus, after farting. The art of "ass bong-ing" or "taking a hit from the ass bong" requires a person to inhale next to the anus as the other farts.

When a frat boy pours beer down the crack of his frat brother's ass while another frat boy drinks it. It's like a beer bong, but instead of a funnel and hose, you use some dude's ass.