One of the varied terms for vomiting. The Lumpy Yawn generally occurs upon waking from a night of hard drinking. Luckily, most foods consumed while "gettin' yer drink on" are soft (hamburgers) or have been generously softened (tacos / nachos) by a cornucopia of beer, and are thusly comfortably ]retched] back up.
Jeremy Clarkson on quitting drinking: "I haven't had a lumpy yawn for a whole week now. Perhaps that's why I'm still fat; I've stopped vomiting"