once in a blue moon
idiom

- not very often

Cambridge Advanced Learner’s Dictionary

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ORIGIN

'A blue moon' refers to a second full moon that occurs in a calender month. Given that a second full moon happens 41 times every century, that means a blue moon occurs about once every two-and-a-half years. In other words, not that often.

It’s not clear what the reference to blue is in this case. However, blue moon once was used to mean “absurd, impossible”. Perhaps the sense of “unlikely” simply evolved from this usage.

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MOON IDIOMS

Our fascination with the moon has led to several other idioms:

- shoot/reach for the moon = set high goals for oneself (Once you get your college degree, just shoot for the moon!)

- ask for the moon = to make outlandish requests or demands (A three percent pay raise is not asking for the moon.)

- promise the moon = to make extravagant promises (Some stores promise the moon in their advertisements, but when you get there they don’t have the product you want.)

- to be over the moon = to be extremely happy about something (John and Mary are over the moon about having their first child.)

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SYNONYMS

almost never, every once in a while, not often, infrequently, hardly ever, infrequently, ONCE IN A BLUE MOON, only now and then, on rare occasions, rarely, scarcely, seldom, sporadically

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SMUGGLE OWAD into a conversation today, say something like:

“ONCE IN A BLUE MOON, I get a sudden urge for chocolate and then I give in to temptation.”

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