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1. Produced by a wildly fanciful imagination
   "his Utopia is not a chimerical commonwealth but a practical improvement on what already exists"
    
2. Being or relating to or like a chimera
   "his Utopia is not as chimerical commonwealth but a practical improvement on what already exists";
   - chimeric, chimeral, chimaeric [Brit]

See also: unrealistic

Encyclopedia: Chimerical