Definition
Indicates that a future event appears uncertain, especially one that before seemed more certain, and where any prior agreements are no longer valid
Anything can happen; any previous preconceptions about what might happen are dismissed.
Origins
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.
How People Actually Use It
an expression meaning a situation in which one factor alone can change or cancel out everything
"When Janine is around Corey, all bets are off...the poor dude just can't be himself anymore."
Source: Urban Dictionary