Bathtub gin refers to any style of homemade spirit made in amateur conditions. The term first appeared in 1920, in the prohibition-era United States, in reference to the poor-quality alcohol that was being made.
As gin was the predominant drink in the 1920s, many variations were created by mixing cheap grain alcohol with water and flavorings and other agents, such as juniper berry juice and glycerin. In addition, mixing grain alcohol, water, and flavorings in vessels large enough to supply commercial users had to be small enough for the operation to go undetected by the police. The common metal bathtub in use at the time would have been ideal as would have been a ceramic bathtub, hence the name, 'bathtub gin'. However, since distillation requires closed distillation apparatus and cannot be accomplished in an open vessel such as a bathtub, stories of distilled alcoholic products produced in an open bathtub are likely untrue.
Many gin cocktails owe their existence to bathtub gin, as they were also created in order to mask the unpleasant taste.
Bathtub Gin is also the name for a gin brand produced by Ableforth's in the United Kingdom. Although not made in a bathtub, it is produced using compounding/infusing rather than using botanical distillation.