expression, noun which may take the genitive case particle 'no'
Used for rare, special opportunities or experiences that may only happen once in one's life. Often modifies nouns with の.
This is a once-in-a-lifetime chance.
I had a once-in-a-lifetime experience.
Compound of 一生 (isshō, 'one's whole life') + に (ni, particle) + 一度 (ichido, 'once'). The literal meaning is 'once in a lifetime'.