Green’s Dictionary of Slang

(US) an establishment that sells illicitly distilled liquor, esp. a private house that does so.

1928		C. McKay Home to Harlem 53: It ain’t no moh than last week they done raided Madame Jerkin’s, the niftiest buffet flat in Harlem.
1929		Pittsburgh Courier 8 Mar. n.p.: This number [of 500 ‘colored cabarets’] is topped by statistics on the apartment speakeasies. Called ‘buffet flats,’ there is an average of two such joints for every apartment building.
1929		T. Gordon Born to Be (1975) 86: In Seattle, the porters would rush to buffet-flats or gambling-houses.
1932		C. McKay Gingertown 36: To encourage and promote intrigues is the prime business of the keep of a buffet flat. Successful intrigues bring good business and new customers.
1965		B. Jackson Get Your Ass in the Water (1974) 123: It’s strange, strange things, how we young fakes can be, / goin’ in these old buffet flats spendin’ our money free. / We don’t ever think about the day we got to fall.
1967		(con. 1925–9) Ottley & Weatherby Negro in N.Y. 249: Many small-time ‘pimps’ and ‘madames’ [...] operated undercover ‘buffet flats’.
1992		(con. 1916-19) M.W. Harris Gospel Blues 51: establishments peculiar to Chicago—‘buffet flats’ [...] Other than for sex, buffet flats were popular because they served as ‘after-hours or Sunday’ drinking establishments.