insanity - definition and synonyms

noun [uncountable] 

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/ɪnˈsænəti/
  1. 1
    old-fashioned very severe mental illness that makes it impossible for someone to be considered legally responsible for their actions

    The judge found him not guilty by reason of insanity.

  2. 2
    informal very stupid or crazy behaviour that can cause serious problems, harm, or injury
    it is insanity to do something:

    It would be insanity to accept their offer.