butt boxing 

When you and your partner fist each other's asshole

Samantha and I had a great butt boxing session last night

butt boxing by Yuumuuuuu February 10, 2014

butt boing 

the act of shoving one's cock up another guy's anus; anal sex

the 2 gays took turns butt boingin' each other

butt boing by Ziggy Stardust February 1, 2004

Buttoxing 

The act of putting ones bare butt against someone when they are not expecting it.

Yo dude i was totally daydreaming and then before i knew it he was buttoxing me.

Buttoxing by buttoxmaster4 December 11, 2011