Green’s Dictionary of Slang

1. (US Und.) money illegally syphoned off, e.g. from a casino’s takings.

2003		F. Wilson Courtesan’s Revenge 338: [endnote] [T]hose in high places who had helped MacGregor pull off his hoax, taking a skim of his earnings in return.
2009		(con. 1960s) J. Ellroy Blood’s a Rover 21: Make sure we get to keep our inside guys, who might just help us out with some skim.

2. the act of defrauding an employer by syphoning off money from the takings.

2012

T.P. McCauley ‘Lady Madeline’s Dive’ in ThugLit Sept./Oct. [ebook] ‘[S]omeone else in this place was in on the skim with you’.