|Origin:||lunatique, from Late Latin lunaticus, from Latin luna ( LUNAR); because people thought mental illness was caused by the moon|
someone who behaves in a crazy or very stupid way - often used humorously:
This hotel is run by a lunatic!
a very offensive word for someone who is mentally ill:
a dangerous lunatic
3 British EnglishPPG
the small group of people in a political group or organization who have the most extreme opinions or ideas
—lunatic adjective [only before noun]