Fiddle / On the fiddle
From the finger movements of playing a fiddle, to fiddle took on the meaning to tamper with something in an aimless, frivolous or idle manner. Fiddle in this sense dates from the mid-1500s. A slang meaning evolved from this, namely to cheat or swindle, which the OED dates from 1604. To be ‘on the fiddle’ means to be involved in cheating or stealing, usually in a petty way. This meaning is more modern and dates from the 19th century.