blue moon
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Tweet 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. [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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