A lunatic is someone who is either clinically insane or just acting really crazy. Someone driving too fast and zigging in and out of traffic is driving like a lunatic.

The root of this word is luna, which means moon. That's because lunatic originally meant someone who went crazy with every phase of the moon, kind of like a werewolf. Most people these days don't believe in moon-caused insanity, but we still talk about lunatics, sometimes meaning clinically insane people. More often this is a slang term, used mainly in exaggerations, for anyone who seems wild and out of control.

Definitions of lunatic
  1. noun
    an insane person
    synonyms: madman, maniac
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    crazy, looney, loony, nutcase, weirdo
    someone deranged and possibly dangerous
    an archaic term for a lunatic
    a person with a mania for setting things on fire
    a woman lunatic
    type of:
    diseased person, sick person, sufferer
    a person suffering from an illness
  2. noun
    a reckless impetuous irresponsible person
    synonyms: daredevil, harum-scarum, hothead, madcap, swashbuckler
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    a reckless and impetuous person
    type of:
    adventurer, venturer
    a person who enjoys taking risks
  3. adjective
    insane and believed to be affected by the phases of the moon
    synonyms: moonstruck
    afflicted with or characteristic of mental derangement
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