insanity n.

1550s, of persons, "mentally damaged," from Latin insanus "mad, insane, of unsound mind; outrageous, excessive, extravagant," from in- "not" (see in- (1)) + sanus "well, healthy, sane" (see sane). In reference to actions, "irrational, evidencing madness," from 1842 in English. The noun meaning "insane person" is attested from 1786. For the notion of insanity as sickness, compare lunatic; and Italian pazzo "insane," originally a euphemism, from Latin patiens "suffering." German verrückt, literally past participle of verrücken "to displace," "applied to the brain as to a clock that is 'out of order' " [Buck].