once in a blue moon
In real, traceable use. Part of the curated seed collection; community review is coming.
Meaning · English
Very rarely; almost never.
Literally: On the rare occasion of a second full moon in a month.
How to use it
✓ Use it when
Emphasizing how seldom something happens.
Register: neutralConnotation: neutralEra: 19th century – present
Where it’s used:
Used
Widespread English
Examples
We only see each other once in a blue moon since she moved abroad.
Editorial example (illustrative, not a corpus attestation)
Sources & notes
Figurative class: Metaphor (PIE-English corpus, CC BY 4.0, retrieved 2026-07-31).
Equivalents & related
Common questions
What does “once in a blue moon” mean?
Very rarely; almost never. Literally: On the rare occasion of a second full moon in a month.
Is “once in a blue moon” formal or informal?
once in a blue moon is generally neutral; connotation: neutral.
What is the closest equivalent?
“năm thì mười hoạ” (Tiếng Việt) — near equivalent.