The Collaborative International Dictionary of English v.0.48:

belly up \bel"ly up`\, belly-up \bel"ly-up`\, a. [from analogy
   to the position of a floating dead fish.]
   defunct; bankrupt; -- used mostly of commercial
   organizations; often used in the phrase

   go belly-up, i. e. to go bankrupt.
      [PJC]