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noun, plural lu·na·cies.
  1. insanity; mental disorder.
  2. intermittent insanity, formerly believed to be related to phases of the moon.
  3. extreme foolishness or an instance of it: Her decision to resign was sheer lunacy.
  4. unsoundness of mind sufficient to incapacitate one for civil transactions: a former legal term.
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Origin of lunacy

First recorded in 1535–45; lun(atic) + -acy


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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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Examples from the Web for lunacy

Contemporary Examples

Historical Examples

  • Here proof of the lunacy of the maiden aunt would be sufficient.

  • But in his red eyes the flame of lunacy was slowly mounting.

    Louisiana Lou

    William West Winter

  • Does not lunacy strike you, Bertie, as being a very eerie thing?

  • It was folly, it was lunacy; but he could not resist it, for he dared not be alone.

    The Scapegoat

    Hall Caine

  • I used to be one of his staunchest supporters, but I draw the line at lunacy.

    Ireland as It Is

    Robert John Buckley (AKA R.J.B.)

British Dictionary definitions for lunacy


noun plural -cies
  1. (formerly) any severe mental illness
  2. foolishness or a foolish act
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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."

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Online Etymology Dictionary, © 2010 Douglas Harper