lu ‧ na ‧ tic /ˈluːnətɪk/ BrE AmE noun [countable]
[ Date: 1200-1300 ; Language: Old French ; Origin: lunatique , from Late Latin lunaticus , from Latin luna ( ⇨ ↑ lunar ); because people thought mental illness was caused by the moon ]
1 . someone who behaves in a crazy or very stupid way – often used humorously:
This hotel is run by a lunatic!
2 . old-fashioned a very offensive word for someone who is mentally ill:
a dangerous lunatic
3 . the lunatic fringe British English the small group of people in a political group or organization who have the most extreme opinions or ideas
—lunatic adjective [only before noun] :
lunatic behaviour