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[loo-nuh-see] /ˈlu nə si/
noun, plural lunacies.
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.
Origin of lunacy
First recorded in 1535-45; lun(atic) + -acy
1. derangement, dementia; craziness, madness, mania, aberration. 3. folly, stupidity.
1, 2. rationality, sanity. Unabridged
Based on the Random House 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?

    The Stark Munro Letters J. Stark Munro
  • 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 (pl) -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
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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
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