The Collaborative International Dictionary of English v.0.48:

bellyacher \bel"ly*ach`er\ n.
   a person who complains habitually, usually about everyday
   minor problems.

   Syn: whiner, complainer, moaner, sniveler, crybaby, grumbler,
        squawker.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bellyacher
    n 1: a person given to excessive complaints and crying and
         whining [syn: whiner, complainer, moaner,
         sniveller, crybaby, bellyacher, grumbler,
         squawker]