Green’s Dictionary of Slang

1. of race, black.

1846

Bell’s Life in Sydney 18 July 3/1: Charlotte, Langley, a sambo angel, applied for protection agains her hard-hearted mate Charles.

2. (US) mixed-race.

1853		Anti-Slavery Bugle (New Lisbon, OH) 21 May 1/2: in the schools referred to were all complexions — Black, White, Mulatto, Sambo.
1870		N.Y. Herald 8 Aug. 3/5: Huddled together were a numbr of black, Sambo and mulatto women and children.