Slang for someone who is addicted to heroin. "Smack" is old street slang for heroin, and "-head" is the piece you add to a drug name to mean a person who cannot stop using it.

The word is not medical and not kind. It sums a whole person up by one thing — thin, sick, not to be trusted, not really a person any more. A doctor or a social worker would say "someone with a heroin addiction", because that puts the person first and the illness second. "Smackhead" does the opposite.

It is mostly British, and it belongs to speech, not writing — you would hear it in a pub or on a TV drama, not in a newspaper report. People also throw it at someone who is simply behaving stupidly or slowly, the way "idiot" gets used: "you smackhead, you locked the keys in the car." That looser use is still rough.

For a learner: understand it when you hear it, and choose the longer, kinder phrase when you speak.