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1. A cruel wicked and inhuman person
   - monster, devil, demon, ogre
    
2. An evil supernatural being
   - devil, demon, daemon, daimon
    
3. A person motivated by irrational enthusiasm (as for a cause)
   "A fiend is one who can't change his mind and won't change the subject";
   - fanatic

Derived forms: fiends

Type of: disagreeable person, enthusiast, evil spirit, partisan, partizan, unpleasant person

Encyclopedia: Fiend