once in a blue moon
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See blue moon.
once in a blue moon (not comparable)
- (idiomatic) Very rarely; very infrequently.
- Synonyms: see Thesaurus:rarely
- 1933, James Hilton, Lost Horizon, published 1973, page 133:
- [T]ime expanded and space contracted, and the name Blue Moon took on a symbolic meaning, as if the future, so delicately plausible, were of a kind that might happen once in a blue moon only.