"Brownbacking is unprotected anal sex, or anal sex without condoms. The term should be understood to refer to risky unprotected anal with multiple and often anonymous partners and a self-identified "brownbacker"—someone with that bumper sticker on his car—is someone who seeks unprotected anal sex with multiple and often anonymous partners. Brownbackers prefer that term because it sounds sexy and healthy and glamorous and it lends itself to brownback-glamorizing bullshit. ... It associates shitty behavior and shitty choices with actual shit."

A jack off buddy (from 'cum' /'come', meaning 'semen', and 'companion'); a friend that you get together with for sessions of solo or mutual masturbation
Variant of: cumpanion (from 'companion' & 'cum'); jack off buddy & wank bud & JO buddy
Similar to: fuck buddy

when you fuck someone so hard in the ass that they shit themselves

Someone that stabs you in the back and then brown noses you to make it back up.