The Collaborative International Dictionary of English v.0.48:

Butterball \But"ter*ball`\, n. (Zool.)
   1. The buffel duck.
      [1913 Webster]

   2. a fat person or animal. [informal]
      [PJC]

   3. a lump of butter rounded into a ball.
      [PJC]

   4. [Trademark] a plump chicken or turkey prepared for
      cooking.
      [PJC]

WordNet (r) 3.0 (2006):

butterball
    n 1: a rotund individual [syn: fatso, fatty, fat person,
         roly-poly, butterball] [ant: scrag, skin and bones,
         thin person]
    2: small North American diving duck; males have bushy head
       plumage [syn: bufflehead, butterball, dipper,
       Bucephela albeola]