Being on the fiddle means being corrupt or getting more than you should from the system. It is a nautical term which refers to the raised edges of the square dinner plates used on board ships. The raised edges (known as fiddles) prevented the food from sliding or rolling off during rough seas. Being on the fiddle meant being given sufficient food to overflow onto the fiddle. Fellow sailors would suspect those given extra food of underhand deals with the ship's staff or cooks.
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