A square meal but are you on the fiddle?

In the British navy of the 18th century, ordinary sailors ate their meals from wooden plates, which were often square rather than round. This gave rise to the phrase a 'square meal' although this may have originally been an ironic comment relating to the poor standard of food. 

The plates were flat but with a raised rim called a fiddle which stopped the food from slipping off. A sailor who pushed his food allowance to the limit, filling his plate so full that food overlapped the rim was said to be 'on the fiddle'.