speakeasy (n.)

"unlicensed saloon," 1889 (in the New York "Voice"), from verbal phrase, from speak (v.) + easy (adv.); so called from the practice of speaking quietly about such a place in public, or when inside it, so as not to alert the police and neighbors. The word gained wide currency in U.S. during Prohibition (1920-1932). In early 19c. Irish and British dialect, a speak softly shop meant "smuggler's den."