blue moon
[noun]
Synonyms: age, aeon, coon's age, dog's age, donkey's years, eternity, long

[TahlilGaran] English Synonym Dictionary ▲

blue moon
n.
►
■ The second of two full moons occurring in the same month.
■ The third full moon in a three-month calendrical season that has four full moons.
► Informal. A relatively long period of time:
I haven't seen you in a blue moon.

[TahlilGaran] American Dictionary ▲

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.]

[TahlilGaran] American Dictionary ▲