Once in a Blue Moon

Originally meant something closer to "never," in the same family as claiming the moon is made of green cheese. Its earliest recorded use is in a 1528 anti-clerical pamphlet by William Roy and Jeremy Barlowe, using it as an example of an obvious absurdity. One theory ties "blue" to the Middle English word "belewe," meaning "betray," connecting it to an extra, calendar-disrupting full moon that forced the medieval church to recalculate the dates of Lent and Easter. The phrase only settled into its modern, much gentler meaning, something rare rather than impossible, by 1821.