Be on the fiddle
Definition 1 of 1
Pro Tip 1/3British Origin
This phrase is more common in British English than American English, so it's useful to know if you're speaking with Brits. 
When I worked in London, I heard a coworker say another was on the fiddle.
Pro Tip 2/3Negative Connotation
The phrase has a negative tone and implies disapproval, so use it in contexts where dishonesty is frowned upon. 
He bought a car that turned out to be on the fiddle - it was stolen!
Pro Tip 3/3Informal
It's an informal idiom, so it's best used in casual conversation, not in formal writing like business or academic papers. 
I wouldn't trust her, she’s known to be on the fiddle.