Insanity noun in-?san-?i-?ty \\ in-'sa-n?-te \\ plural insanities Definition of insanity 1dated: a severely disordered state of the mind usually occurring as a specific disorder 2law: unsoundness of mind or lack of the ability to understand that prevents one from having the mental capacity required by law to enter into a particular relationship, status, or transaction or that releases one from criminal or civil responsibility 3a: extreme folly or unreasonableness the insanity of violence His comments were pure insanity. b: something utterly foolish or unreasonable the insanities of modern life