If something is knobbly, hard little lumps stick out of it, or you can feel hard bits under the surface. The classic picture is knobbly knees — bony knees that are not smooth. Ginger, old tree roots, a walking stick, and some woollen jumpers can be knobbly too. It is a very British spelling; American English more often writes knobby. The word is informal and a little homely, not a doctor's word. Though actually, knobbly is about hard bumps, not about being fat, and not about something soft and lumpy like a cushion.