a)A homosexual, as anal sex is the only form of intercourse for gay men, or
b)An erection in somebody's ass, referring to pulling out and then tapping and wiping the semen on said ass.
Butt-head: I'd like to see you try, buttknocker!
Beavis: Don't call me that, Butt-head! (Beavis kicks Butt-head in the testicles)