We use this expression to make an exception, or to ask for an exception to be made for us.
You can use this phrase in ………… language.
A: I need to leave work early on Friday. Is that possible? An hour early.
B: OK. Just this once.
A: I know I did the wrong thing, but please don’t tell Jana. Just this once.
B: OK – I won’t tell her. But I don’t want to see you doing it again.