Middle English (as potage): from Old French potage 'that which is put into a pot'. Compare with potage and porridge.

• porridge from mid 16th century:

  At first porridge was a soup thickened with barley. The word is a 16th-century alteration of pottage (Middle English), which in turn comes from Old French potage ‘something put in a pot’. The porridge we are familiar with, consisting of oatmeal boiled in water or milk, is mentioned in the 1640s. The informal use of porridge to mean ‘prison’ dates from the 1950s. It probably derives from porridge as a typical prison food, though it might be based on a pun involving two meanings of stir, one as in ‘stir the porridge’ and the other a slang term for ‘prison’, which is perhaps from Romany sturbin ‘jail’. The term was immortalized by the BBC comedy series Porridge of the 1970s, which starred Ronnie Barker as Norman Stanley Fletcher, a cynical but good-hearted old convict.