lunatic
Concise Oxford English Dictionary © 2008 Oxford University Press:
lunatic/ˈluːnətɪk/
▶noun a person who is mentally ill (not in technical use).
■ an extremely foolish person.
– origin ME: from OFr. lunatique, from late L. lunaticus, from L. luna ‘moon’ (from the belief that changes of the moon caused intermittent insanity).
'lunatic' also found in these Oxford entries:
