(used when the outcome of a situation is too uncertain to predict): We have to make it to the meeting place by 5:30 or all bets are off. The individual ingredients may be safe, but once you mix them up into a brew, all bets are off.
The phrase suggests that any previous expectations or outcomes are no longer valid, much like when bets are canceled in a gambling context. It means that the situation has changed so radically that all previous assessments and predictions are no longer reliable. Anything can happen.