Definition of fiendish adjective from the Oxford Advanced American Dictionary

     

    fiendish

     adjective
    adjective
    NAmE//ˈfindɪʃ//
     
    [usually before noun]
     
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  1. 1cruel and unpleasant a fiendish act shrieks of fiendish laughter
  2. 2 (informal) extremely complicated, often in an unpleasant way a puzzle of fiendish complexity a fiendish plan
  3. 3(informal) extremely difficult a fiendish problem
See the Oxford Advanced Learner's Dictionary entry: fiendish