once in a lifetime; extremely important (request)
A set phrase used to emphasize that a request is extremely important or a once-in-a-lifetime opportunity. Often used in the pattern 後生一生のお願い.
This is a once-in-a-lifetime request, so please help me.
The phrase combines 後生 (ごしょう, 'afterlife' or 'future existence') and 一生 (いっしょう, 'one's whole life'), literally meaning 'for the rest of this life and the next'. The exact historical derivation is uncertain, but it is used idiomatically to stress the gravity of a request.