From Longman Dictionary of Contemporary Englishlunaticlu‧na‧tic /ˈluːnətɪk/ noun [countable] 1 CRAZYsomeone who behaves in a crazy or very stupid way – often used humorously This hotel is run by a lunatic!2 old-fashionedMENTALLY ILL a very offensive word for someone who is mentally ill a dangerous lunatic3 → the lunatic fringe —lunatic adjective [only before noun] lunatic behaviour
Examples from the Corpus
lunatic• The boy and girl probably thought he was a lunatic.• Some lunatic came into the store and shot him.Origin lunatic (1200-1300) Old French lunatique, from Late Latin lunaticus, from Latin luna ( → LUNAR); because people thought mental illness was caused by the moon 