blue moon

1. 1.

   a long time

   “something that happens once in blue moon almost never happens”

   Broaderlong time

   colloquialism

Questions about blue moon

What does blue moon mean?

blue moon means "a long time".

Is blue moon a common word?

blue moon is banded "Uncommon" — turns up in careful writing rather than conversation. The band comes from how often the word appears in a large corpus of everyday English, not from an editor's impression.