The Collaborative International Dictionary of English v.0.48:

fatso \fatso\ n.
   a rotund, obese individual.

   Syn: fatty, fat person, roly-poly.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

fatso
    n 1: a rotund individual [syn: fatso, fatty, fat person,
         roly-poly, butterball] [ant: scrag, skin and bones,
         thin person]