From Longman Dictionary of Contemporary English(just) this once(just) this oncespokenONLY used to emphasize that this is the only time you are allowing something, asking for something etc, and it will not happen againGo on, lend me the car, just this once.I’ll make an exception this once. →once
Examples from the Corpus
(just) this once• OK, you can stay up till 11, but just this once.• We've been through this once.• Hadn't she seen something like this once before? she thought vaguely.• But the smiling man who clutched the realtrophy after the game spoke, this once, for everyone.• We've done this once or twice before, as I vividly recall.• Maybe this once, the world will display itself as immutable.• Carol told Petey this once to help him stop crying so she could take a look.• Lawyers and supporters of the parents in Orkney questioned both the motives and the methods of this oncetrusted organisation.• He had swornthis once when he and Adrastus had quarreled and Eriphyle had reconciled them.