Green’s Dictionary of Slang

1. (US black) a black person.

1928

R. Fisher Walls Of Jericho 297: Synonyms of Negro [...] : joker, kack, Mose.

2. (US black) a snobbish person.

1928		R. Fisher Walls Of Jericho 155: Meantime the kacks is closin’ in and you can’t make a quick getaway.
1934		J.L. Kuethe ‘Prison Parlance’ in AS IX:1 27: kack. A refined or nice person.
1949		Monteleone Criminal Sl. (rev. edn).