A state of exhaustion the morning after a night out dancing like crazy. Unlike a regular hangover, this is not necessarily brought on by any alcoholic beverage, but by sheer physical exertion.
A typical sufferer will sleep until late in the afternoon, be fairly sore, especially in the feet, and be ravenously hungry upon waking.
Oh, I'm fine, I was just sleeping off my raver's hangover. Let's go get pancakes!