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.
"Hey dude, you're looking pretty blazed."
"Hell yeah man...I just took a FAT hit from the ass bong..."
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.
Brock: Damn bro, we got wasted last night doing ass bongs!
Brody: I know! Can you believe we did the elephant walk?