Origin of blue moon

First recorded in 1815–25
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British Dictionary definitions for blue moon

blue moon


noun

the second full moon occurring within a calendar month
once in a blue moon informal very rarely; almost never
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012