A song by Alice In Chains off their album Dirt. Written about Layne Staley's heroin addiction.

junk is a slang word for heroin

synonym for the term 'junkie', kind of like calling someone a crack head

The act of getting head in a garbage can. Junkhead can also be utilized as an insult, doing so will insinuate that the person being insulted gets head from a trash can headed bitch.