lunatic

 
Pronunciation: /ˈluːnətɪk/

noun

  • a person who is mentally ill (not in technical use).
  • an extremely foolish or eccentric person: this lunatic just accelerated out from the side of the road

adjective

  • mentally ill (not in technical use): a ward of lunatic old ladies
  • extremely foolish or eccentric: he would be asked to acquiesce in some lunatic scheme

Origin:

Middle English: from Old French lunatique, from late Latin lunaticus, from Latin luna 'moon' (from the belief that changes of the moon caused intermittent insanity)