The phrase comes from watching a dying fish. When fish die, they often float upside down with their belly up. If something starts to go "belly up", it will not be long before it is no longer functional.

When an animal dies, it goes belly up. The same thing happens with our plans.

This phrase is informal. "To go belly up" means to stop succeeding in something, to have a complete failure.

means falling apart

to go belly up = to undergo total financial failure: "A record number of banks went belly up" Last week people's shares went down and things started to go belly up .