Gruel is a truly unpleasant food — weak and runny, consisting of oatmeal or cornmeal boiled in milk or water. It's the kind of "slop" prisoners and other inmates of institutions were historically forced to eat.
Gruel was made most famous by Dickens's Oliver Twist, the little orphan boy in the workhouse, who was so hungry he even asked for seconds of it: "Please sir, I want some more." Gruel's reputation, not great to begin with, never recovered. From this delightful substance comes the adjective grueling, describing an experience that's exhausting and punishing. "To get one's gruel" was 1700s slang meaning "to receive one's punishment." Even back then gruel had a bad rap.