blue moon

blue moon

n.

1.

a. The third full moon in a three-month calendrical season that has four full moons.

b. The second of two full moons occurring in the same month.

2. Informal A relatively long period of time: I haven't seen you in a blue moon.

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[Sense 2, probably from the rare occurrence whereby the moon appears blue from high amounts of dust in the atmosphere, as from a volcanic eruption.]

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blue moon

n

1. the second full moon occurring within a calendar month

once in a blue moon Informal very rarely; almost never

Thesaurus

Noun

1.

blue moon - a long time; "something that happens once in blue moon almost never happens" colloquialism - a colloquial expression; characteristic of spoken or written communication that seeks to imitate informal speech long time, years, age - a prolonged period of time; "we've known each other for ages"; "I haven't been there for years and years"