And the answer: bootlegger.
It’s Tom Buchanan who states in Chapter 7: “I picked him for a bootlegger the first time I saw him, and I wasn’t far wrong.” Gatsby and Meyer Wolfsheim bought drug stores both in New York and Chicago, where they sold grain alcohol over the counter during the Prohibition era.
As the name implies, the term "bootlegger" came into use when boots were used to conceal illicit flasks of alcohol during trades between settlers and Native Americans in the 1880s midwest. Then, when the 18th amendment was ratified and the sale of alcohol effectively prohibited in the United States, the term only grew. The practice of bootlegging took off throughout the 1920s, when Americans would consume illegal alcohol in bars known as speakeasies. Nearly all of these speakeasies were supplied, in some form, by bootleggers.
Even though alcohol was illegal in the United States, it was not illegal in Canada, Mexico, or the islands of the Caribbean sea. Thus, bootleggers would smuggle alcohol across the border in trains, trucks, ships, and even on foot. To maintain the utmost secrecy of the illegal affair, bootleggers would pack the alcohol into crates with false labels, hide it underneath legitimate products, or even stash the alcohol inside items such as coconuts.
As prevalent as the practice of smuggling alcohol was, bootleggers required code words to refer to their product without getting busted by authorities. Thus, slang terms such as booze, hooch, devil's candy and bathtub gin were all used to refer to alcohol during the prohibition era.
Learn more about the history and significance of bootleggers here.