A fiendish plan, action, or device is very clever or imaginative. a fiendish plot. = devilish + fiendishly fiend·ish·ly This figure is reached by a fiendishly clever equation
extremely evil or cruel; expressive of cruelty or befitting hell; "something demonic in him--something that could be cruel"; "fires lit up a diabolic scene"; "diabolical sorcerers under the influence of devils"; "a fiendish despot"; "hellish torture"; "infernal instruments of war"; "satanic cruelty"; "unholy grimaces"
A fiendish problem or task is very difficult and challenging. the fiendish difficulty of the questions. + fiendishly fiend·ish·ly America's trade laws are fiendishly complex