noun
a crazy or mentally unstable person
adjective
crazy or mentally unstable
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The psychiatrist diagnosed the patient as a loony individual suffering from a mental disorder.
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I heard that guy is a bit loony, always talking to himself and laughing for no reason.
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She's a total loony tune, always doing crazy things to get attention.
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The idea of quitting your job and traveling the world may sound loony to some, but to others, it's a dream come true.