wʌns ɪn ə bluː muːn
Nature & Weather
informal
beginner
- Literally:
- When the moon appears blue
What does "once in a blue moon" mean?
- Very rarely; almost never
- Happening at extremely long intervals
Examples
- I only see him once in a blue moon.
- She eats junk food once in a blue moon.
- Opportunities like this come once in a blue moon.
Origin
A 'blue moon' is the second full moon in a calendar month—a rare occurrence. The expression dates to the 19th century.
How to use it
Emphasizes extreme rarity. Common and widely understood.
Similar expressions
rarely
seldom
very infrequently
hardly ever
Opposites
frequently
often
regularly
Related idioms
- out of the blue
Unexpectedly; without warning
- beat around the bush
To avoid talking about the main point of a conversation
- every cloud has a silver lining
There is something positive in every negative situation
- go with the flow
To accept things as they are and adapt accordingly
- tip of the iceberg
A small visible part of a much larger hidden problem
- it's raining cats and dogs
It's raining very heavily
Use it in a practice answer
Try "once in a blue moon" in a CELPIP-style Speaking or Writing task. The AI feedback tells you whether your word choice worked.