Green’s Dictionary of Slang

1. a derog. term for a black person, esp. one with a notably dark complexion.

1842		Whip & Satirist of NY & Brooklyn (NY) 28 May n.p.: One of the Congo breed of Orang Otangs of whom there is a great many at present in the city.
1857		Perrysburg Jrnl (OH) 8 Oct. 4/1: Congo Cox [...] says: ‘All Distinctions of caste are odious!’ Stuff that up your delectable, fastuidious Congo noses! Retire to your polecat kennels and lie down in the same lair with Pompey, Sambo and Julius Caesar!
1877		F.A. March Anglo-Sax. Grammar (1883) 36: There may be as many genders as there are sets of terminations...the Congoes and Caffirs have many.
1983		I.L. Allen Lang. of Ethnic Conflict 48: Allusions to African Origins: congo [1760].
1994		I. Welsh ‘Stoke Newington Blues’ in Acid House 38: We’re all white men. Do time for some bleeding Congo? Wise up, Jock.

2. attrib. use of sense 1.

1865		Leaves from Diary of Celebrated Burglar 9/2: Mouth extremely large, with Congo lips.
1906		S. Ford Shorty McCabe 93: Mr. Rufus Rastus, the Congo brunet that’s master of ceremonies on the car.
1953		W. Brown Run, Chico, Run (1959) 20: Congo Dewey came down from Harlem to collect for the policy racket.