once in a blue moon

/wʌns ɪn ə bluː mun/◆idiom

very rarely

I don't eat fast food, but once in a blue moon I'll grab a burger.

The speaker almost never eats fast food, with this being an extremely rare exception.

◆ What It Really Means

an event that occurs extremely rarely, almost never

¶ Literal Meaning

one time during the occurrence of a blue moon

Literal Breakdown

onceone instance+inwithin the period of+a blue moona rare astronomical event where the moon appears blue (though it typically does not)

◇ Mental Image

A mental picture of a rare and unusual event, like a blue-colored moon appearing in the sky, which is so infrequent that it stands out as extraordinary.

◈ When to Use

Imagine someone saying, 'I go to the gym once in a blue moon.' This implies they almost never exercise, with gym visits being as rare as a blue moon in the sky.

◉ Cultural Note

The phrase is deeply embedded in English-speaking cultures, often used humorously or exaggeratedly to emphasize how seldom something happens. It reflects a cultural appreciation for rare and unusual events, even if they are not scientifically accurate.

informal

very rarely; almost never

She visits her hometown once in a blue moon, maybe once every five years.

She rarely visits her hometown, with visits occurring only about every five years.

I only watch horror movies once in a blue moon because I get scared easily.

The speaker almost never watches horror movies due to fear.

Used to emphasize the infrequency of an event. The phrase does not imply a specific time frame, only that the event is highly unusual.

Collocations

happens once in a blue moonoccurs very rarelysee someone once in a blue moonrarely encounter someone

Synonyms

rarely seldom hardly ever once in a while (but much less frequently)not often

Antonyms

often frequently all the time regularly

Related Phrases

once in a lifetimeidiom

an event that happens only once in one's life

every now and thenphrase

occasionally, but not frequently

once and for allphrase

finally and conclusively

Pro Tip

Avoid literal interpretation

Do not confuse this idiom with the actual astronomical event of a blue moon. The phrase is purely figurative and should not be taken literally.

Gold Rule

Use for emphasis on rarity

This idiom is best used when you want to strongly emphasize how infrequently something occurs. It is not suitable for events that happen moderately often.

Word Origin

The phrase likely originates from the rare astronomical phenomenon of a 'blue moon,' which is the second full moon in a calendar month. This event occurs approximately once every 2.7 years, making it a fitting metaphor for rarity. The term 'blue moon' itself dates back to at least the 16th century, while the idiomatic use of the phrase emerged in the early 19th century.

Usage Notes

This idiom is used in informal and conversational contexts to describe actions or events that happen very infrequently. It is not suitable for formal writing or technical contexts. The phrase is always used adverbially to modify a verb.

Word Breakdown

once

one time

adverb

during the course of

preposition

one

article

blue moon

a rare event

noun phrase

English Dictionary

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