Definition of lunatic noun from the Oxford Advanced American Dictionary



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  1. 1a person who does crazy things that are often dangerous synonym maniac This lunatic in a white van pulled out right in front of me!
  2. 2(old-fashioned) a person who is severely mentally ill (the use of this word is now offensive) Originally from the Latin lunaticus (luna = moon), because people believed that the changes in the moon made people go mad temporarily.
See the Oxford Advanced Learner's Dictionary entry: lunatic