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)