once in a lifetime

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once in a lifetime

Describing something, such as an opportunity, experience, or situation, that seems unlikely to happen again. Often hyphenated when used as a modifier before a noun. You have to take this job in the Zurich office—it's a once-in-a-lifetime opportunity to live in Switzerland. Chances like this only ever come around once in a lifetime, Jack, so don't waste it.
See also: lifetime, once

once in a lifetime

Extremely rare, especially as an opportunity. For example, An offer like that will come just once in a lifetime. This phrase, often used hyperbolically, was first recorded in 1854. Also see of one's life.
See also: lifetime, once