Green’s Dictionary of Slang

gin-mill n.

also whiskey-mill
[SE gin + mill + pun on gin, a type of mill]
(US)

1. [mid-19C+] a bar or nightclub, orig. a speakeasy specializing in cheap, and prob. adulterated, liquor; thus gin-millist, a bartender; gin-mill row, a street of bars.

2. [1960s] a liquor store.