What was the origin of "boiler room gin"?

Bathtub gin came about during Prohibition in the US. Since selling liquor was illegal, people would make their own at home. Whether they did so in the bathtub is unclear, although the equipment for distilling gin is pretty big.
"Boiler room investing" is a term for any fraudulent scheme to sell worthless stocks and bonds to unsophisticated investors. There would be a bank of telephones and the criminals would just call people and offer a get rich quick scheme. The investors got nothing but a phony piece of paper and maybe fake monthly statements..

A boilermaker can refer to two types of beer cocktail. In American terminology, the drink consists of a glass of beer and a shot of whiskey. The beer is either served as a chaser or mixed with the whiskey. When the beer is served as a chaser, the drink is often called simply a shot and a beer. In Philadelphia, it is commonly referred to as a Citywide Special; in Texas, it is known as a Two-Step, and, in parts of Florida, it is often referred to as a Git-Right.[citation needed]
The drink originated in Butte, Montana, in the 1890s, and was originally called a "Sean O'Farrell" and was served only when miners ended their shifts.
In England, the term "boilermaker" traditionally refers to a half pint of draught mild mixed with a half pint of bottled brown ale, although it now also commonly refers to the American shot and pint. In Scotland, "a half and a half" is a half pint of beer with a whisky ("a hauf"). The use of these terms in Scottish and English pubs can be traced back to about 1920.