blue moon
noun
Definition of blue moon
1 : a very long period of time —usually used in the phrase once in a blue moon such people happen along only once in a blue moon — Saturday Rev.
2 : a second full moon in a calendar month
From the M-W Editors
Examples of blue moon in a Sentence
once in a blue moon we'll get dressed up and eat in a fancy restaurant
1821
First Known Use of blue moon
1821
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