one shot in your locker
one shot in (one's) locker
old-fashioned A single, final opportunity or chance to be successful. We only have this one shot in our locker—if this doesn't cure your husband's illness, then I'm afraid we'll have to prepare for the inevitable.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
one shot in your locker
If you have only one shot in your locker, you have only one thing left that you can do in order to succeed. He had just one shot left in his locker and if that failed, he would be ruined. Note: A locker is a small cupboard with a lock. In this case, it might be a cupboard containing ammunition.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012