Green’s Dictionary of Slang

a black person.

1768		I. Bickerstaffe Padlock I vi 11: What e’er’s to be done, Poor black must run; Mungo here, Mungo dere, Mungo every where. Above and below, Sirrah come, sirrah go.
[1774		E. Long Hist. of Jamaica II 427: Negroe’s name, Mundingo].
1794		Sporting Mag. Feb. III 289/2: Blackey fired a pistol at his pursuers, for which he was taken up and committed [...] Mungo was taken by a press gang, and put on board the tender.
1804		Sporting Mag. Jan. XXIII 216/2: Mungo could not endure any more tumpers.
1821		Egan Life in London (1869) 290: The little mungo in the corner, holding his arms out, is her child.
1842		Comic Almanack Nov. 336: Hark! Mungo speaks – ‘O golly! what a go, / Them four-um-twenty bottles ob a row [...] Come, massa butler! neber spare um whack; / Mungo shall drink, so long as Mungo black.’.
1879		‘A Black Joseph’ in Pearl 5 Nov. 24: Mungo – according to his own account – avoided going near her as much as possible.