Blue moon
WordNet

noun


(1)   A long time
"Something that happens once in blue moon almost never happens"
WiktionaryText

Noun



  1. The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere.
  2. Something absurd.
    • 1528, Rede Me and Be Not Wrothe, pamphlet
      Yf they say the mone is belewe, we must believe that it is true.
  3. The third full moon in a quarter that contains four rather than the usual three full moons.
  4. A long time.

Noun



a blue moon
  1. The second of two full moons that occur in the same calendar month.
 
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