Entry from OED Online

[Origin unknown.] 

    Money.
 
  1953 H. CLEVELY Public Enemy xviii. 114 He hadn't enough dosh on him. 1959 I. & P. OPIE Lore & Lang. Schoolch. ix. 155 Money..is referred to as..moolah, dosh (common), sploosh, [etc.]. 1970 M. KENYON 100,000 Welcomes xvi. 139 ‘America! The money's in America!’..‘'Tis true. The Yankees have the dosh all right.’