A speakeasy (one word) was a secret “black market” bar during the American Prohibition of alcohol. Because they traded in illegal products, they had to restrict how many people knew of the speakeasy’s existence and location so the police wouldn’t find out about it. So whenever somebody spoke of the bar in public, they had to “speak easy” or softly, so the wrong people wouldn’t overhear.
The speakeasy got its name because one had to whisper a code word or name through a slot in a locked door to gain admittance.