[ loo-nuh-see ]
/ ˈlu nə si /

noun, plural lu·na·cies.

insanity; mental disorder.
intermittent insanity, formerly believed to be related to phases of the moon.
extreme foolishness or an instance of it: Her decision to resign was sheer lunacy.
unsoundness of mind sufficient to incapacitate one for civil transactions: a former legal term.

Nearby words

  1. lumpy jaw,
  2. lumumba,
  3. lumumba, patrice emergy,
  4. luna,
  5. luna moth,
  6. lunar,
  7. lunar caustic,
  8. lunar cycle,
  9. lunar day,
  10. lunar distance

Origin of lunacy

First recorded in 1535–45; lun(atic) + -acy Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for lunacy

British Dictionary definitions for lunacy


/ (ˈluːnəsɪ) /

noun plural -cies

(formerly) any severe mental illness
foolishness or a foolish act
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for lunacy



1540s, "condition of being a lunatic," formed in English from lunatic + -cy. Originally in reference to intermittent periods of insanity, such as were believed to be triggered by the moon's cycle. The Old English equivalent was monaðseocnes "month-sickness."

Online Etymology Dictionary, © 2010 Douglas Harper