We use this expression to say that you have spent or lost money when you didn't expect to.
We've had hardly any customers in the shop this Sunday.
Well I still have to pay the staff so I guess we'll be out of pocket again.
Did the bus company pay you compensation for arriving so late?
Yes they paid me back the full cost of the ticket. But they won't pay me for the concert that I missed. So I'm still out of pocket.