blue moon
Idioms
Origin of blue moon
First recorded in 1815–25
Words nearby blue moon
blue melilot,
blue merle,
blue mockingbird,
blue mold,
blue monday,
blue moon,
blue mould,
blue mountains,
blue mud,
blue murder,
blue myrtle
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