One shot in (one's) locker old-fashioned noun phrase
One shot in (one's) locker is the last opportunity for someone to successfully achieve something.
This game is one shot in our locker, so try our best to reverse the situation.
Tonight he will fight as if it's the one shot in his locker.
Nothing but fortunate chance
If you take a flyer (on something), you take a chance, risk, or gamble on it.
Having only one chance left one can do in order to achieve something
A second opportunity
Very little chance of succeeding in doing something
Locker is a drawer, cupboard, or compartment usually seen at gymnasiums, schools that may be closed with a lock. Bullets/ammunition might be stored in it in this case (the word "shot"). Maybe that's why we have this idiomatic expression.
Used to allude that the last force, problem or burden which is seemingly minor and small causes a person, system or organisation to collapse or fail
Her husband's violent act last night was the straw that broke the donkey's back and she left him