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.
Law. unsoundness of mind sufficient to incapacitate one for civil transactions.

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. 2014.
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World English Dictionary
lunacy (ˈluːnəsɪ)
n , pl -cies
1.  (formerly) any severe mental illness
2.  foolishness or a foolish act

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1540s, "condition of being a lunatic," formed in English from lunatic (q.v.). Originally in ref. to intermittent periods of insanity, such as were believed to be triggered by the moon's cycle. The O.E. equivalent was monaðseocnes "month-sickness."
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
His descent into madness is rapid but not sudden, and his lunacy seems
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.
Related Words
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