Green’s Dictionary of Slang

(US black) the Atlantic Ocean.

1943		D. Burley N.Y. Amsterdam News 11 Dec. 6B: Between us and the forty-eight lays the big moist.
1944		D. Burley Orig. Hbk of Harlem Jive 123: The Squares ’cross the Big Moist are booted to the fact that they can’t win.