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
Primarily refers to baatezu, gehreleths, hordlings, tanar'ri, and yugoloths, but sometimes includes any intelligent being native to the Lower Planes (night hags, imps, quasits, and the like)
a person motivated by irrational enthusiasm (as for a cause); "A fanatic is one who can't change his mind and won't change the subject"--Winston Churchill
If you describe someone as a fiend, you mean that they are extremely wicked or cruel. A tearful husband repeated calls for help in catching the fiend who battered his wife. = monster
Fiend can be used after a noun to refer to a person who is very interested in the thing mentioned, and enjoys having a lot of it or doing it often. if you're a heavy coffee drinker or strong-tea fiend