Green’s Dictionary of Slang

1. 10%.

1990

J. Ellroy ‘Gravy Train’ in Pronzini & Adrian Hard-Boiled (1995) 490: I take a tensky off the top to cover your bill.

2. a $10 bill.

1986		J. Ellroy Silent Terror 56: [T]hey've got a pad off the Strip, and they spread all night for a tensky.
1996–2003		I.T.S. ‘The Goldstein Report’ on Handicappers Daily 🌐 There are many other venues covered all for a minuscule $9.95 per month. I’m not involved with the site or company but will soon be a subscriber. I can’t speak to quality of the coverage as I’ve only seen the preview on the site, but it will only cost less than a tenski to find out.