Originally used by sports bookmakers, and in particular horseracing. The bookmaker is not taking any more bets and all existing bets placed on the competition are now null and void. This may have been because the race was cancelled, the starting lineup was substantially changed, or due to an irregularity in the conduct of the race.
- (idiomatic) Indicates that a future event appears uncertain, especially one that before seemed more certain. If all bets are off, then any prior agreements are no longer valid.
- By extension, "anything can happen"; any previous preconceptions about what might happen are dismissed.