The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
blue moon
NOUN:
1a. The second of two full moons occurring in the same month. b. The third full moon in a three-month calendrical season that has four full moons. 2.Informal A relatively long period of time: I haven't seen you in a blue moon.
ETYMOLOGY:
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.
