One of the dozens of slang terms used by Beavis and Butt-head. Buttknocker could refer to
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.
Beavis: Shut up, Butt-head! I'm gonna play drums on your nads with my foot!
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)
1.) Term made popular by Beavis and Butthead.
2.) One who exists to knock on ass. Arguably an accusation of homosexuality.
Beavis: Heheheh, move bunghole!
Butthead: Uh, shut up buttknocker! Uhuhuhuh.