1. graded adjective & adjective [usu ADJ n] A fiendish plan, action, or device is very clever or imaginative. [informal] ⇒ ...a fiendish plot. fiendishly
adverb [usu ADV adj] ⇒ This figure is reached by a fiendishly clever equation. 2. graded adjective & adjective [ADJ n] A fiendish problem or task is very difficult and challenging. [informal] ⇒ A rather neat option allows you to design your own fiendish puzzle. ⇒ ...the fiendish difficulty of the questions. fiendishly
adverb [ADV adj] ⇒ America's trade laws are fiendishly complex. 3. graded adjective & adjective [usu ADJ n] A fiendish person enjoys being cruel. ⇒ This was a fiendish act of wickedness.
COBUILD Advanced English Dictionary. Copyright © Harper Collins Publishers