Wiktionary
n. (context idiomatic English) gin or a similar alcoholic beverage which is of very poor quality, as if made by a homebrewer in a bathtub.
WordNet
n. homemade gin especially that made illegally
Wikipedia
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. Because the preferred sort of bottle was too tall to be topped off with water from a sink, they were filled from a bathtub tap. 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'. Note: Distillation requires closed distillation apparatus and can not be accomplished in an open vessel such as a bathtub and so stories of distilled alcoholic products produced in an open bathtub are very 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. Whilst not being made in a bathtub it is produced using compounding/infusing rather than using botanical distillation.
Usage examples of "bathtub gin".
Some served hard stuff as well as beer, and by all accounts I ever heard, the stuff you could get in town was ten times as good as the rotgut whiskey and bathtub gin you could get at the white boys' NCO on Friday and Saturday nights.
Whole lot of whoopee going on back then, flappers flapping, drunk on bathtub gin, ladies holding on to their brimless cloche hats as they indulged in the new sport of motoring.
There was also the matter of the bathtub gin served in green squeeze bottles.
He quickly filled each glass with what looked to Hunter to be bathtub gin.
And after the Ekhemasu came to understand the effect of ethanol on the human nervous system, they produced a kind of bathtub gin that was more or less passable.
Comparatively, they had it easy in the old prohibition days -- bulk smuggling from Canada, bathtub gin, illicit stills by the thousand.