on the fiddle的含义不是“在小提琴上”，其正确的含义是：用欺骗的手段（或诡计）获取利益；搞骗钱勾当；干骗钱的勾当;诈骗。

【例句】
My career will be over if anyone ever finds out I was on the fiddle during my time as the company treasurer.
如果有人发现我在担任公司财务主管期间用欺骗的手段获取利益，我的职业生涯就会结束。

He was on the fiddle for years and his boss never suspected a thing.
他用欺骗的手段获取利益很多年，他的老板从未怀疑过。