What does ‘once in a blue moon’ mean?
To “once in a blue moon” means to very rarely. Brewer's records the astronomical and folk background: a 'blue moon' referred both to unusual atmospheric appearances (as after a volcanic eruption) and to a second full moon in a calendar month, both rare events.
Origin
- Brewer's records the astronomical and folk background: a 'blue moon' referred both to unusual atmospheric appearances (as after a volcanic eruption) and to a second full moon in a calendar month, both rare events.
How to use it
- Common casual measure of infrequency.
- Example: I only see my old classmates once in a blue moon.
Source:
Last verified: 2026-07-18
- Definitions and origins are drawn from public-domain reference works, primarily Brewer's Dictionary of Phrase and Fable (1898), with modern usage notes clearly marked.