Someone who manufactures illicit drugs in their back room, usually on a desk or in a bathtub. The conditions these drugs are made and stored in are usually very dirty and the drugs made are usually cut with various other substances (everything from cheaper drugs to talcum powder to bleach to rat poison), either because they have limited knowledge of drug manufacture and chemistry or (more likely) because they couldn't care less about the health and well-being of their prospective customers. They instead prefer to use less (or none at all) of the advertised drug per pill/dose and therefore increase profit.
A guy sold me a pill last night. He told me it was Ecstasy but it was mixed with all sorts of shit and I ended up in hospital. I'm sick of these bathtub chemists ripping me off!!