One of the many terms for a heroin addict. None of the slang is complimentary, reflecting the devious and desperate lives that addicts experience. The next bag, or the next tenner to buy that bag, is the sole object of their existence. Morality, decency, all that makes us human appears to be relegated to being a sliver of their consciousness.
This was only brought home to me when I was sitting out on the landing with Tony. Tony was 'the man', one of a handful in the nick who had large reserves of cash and cannabis.
A smackhead arrived. He wanted to borrow twenty quid from Tony. He wasn't having much luck; Tony didn't know him and treated all smackheads as poor financial risks.
Hard bargaining ensued. The smackhead eventually left with his money, having sold his trainers and spectacles to Tony. He ran back to his mates, cash in hand, barefooted and half blind - but very happy. It made me shudder.