Longman Dictionary of Contemporary English homepage

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

lunatic

 noun
     
lu‧na‧tic [countable]
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 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]
lunatic behaviour
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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