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)
Royal Marine (British Special Forces). Known to each other as "Royal" but the rest of the British Forces know them as bootnecks. This nickname is derived from them cutting the top from a leather boot and wearing it like a modern neckbrace to stop sailors cutting their throat while they guarded officers on board British sailing ships in the days of old.
"Don't anoy those Bootnecks over there unless you want to wake up with a crowd round you!"