Picture a little devil leaning over a desk, working something out and enjoying it. That figure has three things about it: it means harm, it is very clever, and whatever it makes is hard for everyone else. English takes one of the three each time it uses this word. Sometimes only the harm is left, sometimes only the cleverness, and sometimes only the designed difficulty — which is why a fiendish crime, a fiendish plan and a fiendish can all come from the same word without each other.
From the word’s history
Fiendish is simply fiend plus the suffix -ish, the same -ish as in childish and foolish, meaning 'like a ...'. So the word literally says 'like a fiend', and a fiend is a devil or evil spirit. That is why every use still keeps some part of the devil figure, whether it is the malice, the cunning, or the sense that the difficulty was put there on purpose.
fiendish 就是 fiend 加上字尾 -ish，跟 childish、foolish 是同一個 -ish，意思是「像⋯⋯一樣」。fiend 是惡魔、邪靈，所以字面就是「像惡魔一樣的」。這也是為什麼它的每一個意思都還留著惡魔身上的一部分：有時候是惡意，有時候是精明，有時候是「這難度一定是有人故意做出來的」那種感覺。
Wiktionary etymology supplied in this evidence packet (kaikki:3f6f69a190036ae4:570196548:11431): fiend + -ish (Old English -isċ < Proto-Germanic *-iskaz).
登入就可以看每一支怎麼連回核心畫面。