Green’s Dictionary of Slang

cue n.²

[? mid-15C cue or q (Lat. quadrans), half a farthing]

(US black) a tip.

1960, 1975		Wentworth & Flexner DAS.
1964		R. Abrahams Deep Down In The Jungle 265: Cue – Used here to mean tip in the sense of money given for services.