once in a blue moon
- 1Very rarely: only every now and then, almost never.
Like seeing a shooting star: it almost never happens.
We eat out only once in a blue moon.
My uncle visits us once in a blue moon, usually around the holidays.
Similarrarely, seldom, infrequently, scarcely, uncommonly
Oppositeoften, frequently, regularly, constantly
Note
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Origin
It comes from the phrase "blue moon" and was first used around 1833.