Green’s Dictionary of Slang

1. a violin.

1944		Maurer & Baker ‘“Aus.” Rhyming Argot’ in AS XIX:3.
1960		J. Franklyn Dict. of Rhy. Sl.
1984		Partridge DSUE (8th edn) 617: [...] C.20.

2. (Aus.) a drink ‘on the house’ [the implication that the publican is ‘fiddling’ himself out of a profit].

1941		Baker Popular Dict. Aus. Sl.
1955		N. Pulliam I Travelled a Lonely Land (1957) 235/1: jerrydiddle a drink on the house.