No, it's not what you get for picking up the wrong pint in the The Mended Drum. The knuckle sandwich is to Ankh-Morpork as cheese steak is to Philadelphia or a seafood poorboy to New Orleans. It is the local soul food : older than pizza and infinitely preferable to CMOT Dibbler's sausages.
It is basically a pulled-pork sandwich of steamed or boiled pigs' knuckles, shredded on crusty bread, usually with mustard, and likely washed down with a pint of Winkles. There is a very good recipe in Nanny Ogg's Cookbook which tells you more than you need to know about porcine hooves.