The story
Variant of pottage (“thick soup or stew”), influenced by porray (“stew of leeks”). The "prison sentence" sense comes from the British tradition of serving prisoners porridge for breakfast.
Etymology from Wiktionary, CC BY-SA 4.0.
porridge
Today it means: A dish made of grain or legumes, milk or water, heated and stirred until thick and typically eaten for breakfast. Full definition →