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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
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. 2015.
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Examples from the web for lunacy
  • His descent into madness is rapid but not sudden, and his lunacy seems inevitable.
  • The first is excellent on the cosmology and physics interface and the second is great at debunking pseudoscience lunacy de jour.
  • Extending into the private sector a policy that has been a disaster in the public sector is lunacy.
  • Our government's policies on drugs are sheer lunacy.
  • In fieldwork, strangers thrown together often find common ground in bouts of functional lunacy.
  • The idea that one method fits all is a bit of lunacy.
  • It is lunacy to submit our children and the children of our neighborhood to preventable diseases.
  • The quotes are not implausible, it is the interpretation that is lunacy.
  • All three are always there, but it is the last that pushes lunacy in the other two.
  • lunacy in a family is not a particularly funny thing, nor does it seem fitting and tasteful as a matter to be treated as farce.
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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