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blue moon Definition

blue moon

noun

a very long time

Etymology: < a rare atmospheric condition in which layers of smoke, dust, etc., as from forest fires, volcanoes, etc., cause the surface of the moon to appear to be blue

blue moon Idioms

once in a blue moon

very seldom; almost never

blue moon Usage Examples

Adjective modifier

last: The last blue moon was in May 2002, the next is in December 2005.