Lumpy things are uneven and full of bumps or chunks. While lumpy oatmeal might be good, a lumpy mattress can make it impossible to get a good night's sleep.
It's easier to play a game of croquet on a smooth lawn, rather than a lumpy field full of bumps and holes. And you may find a silky bowl of chocolate pudding delicious, but be disgusted by the texture of a lumpy serving of tapioca pudding. The adjective lumpy, by way of the noun lump, comes from the Old English lumpe, from a Scandinavian root.