Green’s Dictionary of Slang

1. (US black) a black area of a US city.

1943		D. Burley N.Y. Amsterdam News 11 Dec. 6B: [of Harlem] The cruit-suited studs here who hail from the Apple’s Congo.
1967		R. Conot Rivers of Blood 38: [H]is fellow officers [...] kidded him about being transferred to ‘the L.A. Congo’.

2. (drugs, also Congolese bush) marijuana from the Congo area of Africa [SE Congo + bush n.¹ (5a)].

1966		T. Keyes All Night Stand 121: I’ve got this very good congo, not cut, ya know.
2002		Fabian & Byrne Out of Time (ms.) 91: When I discovered the next train wasn’t for another hour and that Crim had some Congolese Bush I agreed to go to Regent’s Park for a smoke on the way.