"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."
As a Brownbacker, a guy will often end up with santorum all over his dick.