Green’s Dictionary of Slang

stealing from the till, taking money ‘off the top’, esp. as found in casinos, strip clubs and other places where a degree of criminality is already endemic.

1971		E. Wilson Show Business Nobody Knows 42: ‘[S]kimming’—the process of removing a few thousand in the [casino] counting room before turning in the ‘final figures’.
1983		A-Team Storybook 50: The information it contained about kickbacks, protection, fund skimming, and political pay offs.
1997		C. Hiaasen Lucky You 103: Whatever true sum was lost in the Simmons Wood venture would be doubled when it appeared as red ink on the books [...] That is how Bernard Squires hid the Tarbone family’s skimming.