A song by Alice In Chains off their album Dirt. Written about Layne Staley's heroin addiction.
junk is a slang word for heroin
fan 1: yo man, did you get Dirt by Alice In Chains yet?
fan 2: yeah, did you hear Junkhead?
fan 1: yeah, its about Layne Staley and heroin
synonym for the term 'junkie', kind of like calling someone a crack head
Honey, look at that junkhead shooting up heroin over there.