Green’s Dictionary of Slang

1. (US black) a second-rate, inferior person; used as an insult.

1928

R. Fisher Walls Of Jericho 94: Don’t a man move! – knew I’d turn the bug on you dinkies this time! And he threw down an ace and a jack, the supreme combination.

2. (US, also dinkey) a black person, usu. derog.

1928		R. Fisher Walls Of Jericho 297: Synonyms of Negro [...] : Cloud, crow, darky, dinge, dinky.
1945		Mencken Amer. Lang. Supplement I 634: Dinkey, in the Baltimore of my nonage, meant a colored child.