once in a lifetime

US /wʌns ɪn ə ˈlaɪfˌtaɪm/
UK /wʌns ɪn ə ˈlaɪfˌtaɪm/
"once in a lifetime" picture
1.

an event or opportunity that is very rare and unlikely to happen again in a person's life

:
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Seeing the total solar eclipse was a once in a lifetime experience.
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This opportunity to travel the world is truly a once in a lifetime chance.
:
lifetimeoncebreak the habit of a lifetimeonce and for allonce upon a timeonce in a blue moongive somebody/something a once-over