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.
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).
Two branches
像惡魔的人與心思
這一家離核心畫面最近：字面上的「像惡魔一樣」。兩個意思的差別，只在於取的是惡魔的哪一半——狠，還是精。
adjective · C1
Cruel and unpleasant, in a way that seems to enjoy causing harm or trouble; it is used about people, and about the things they do, such as a trick, a punishment or a crime.
狠毒的／惡劣的
難到像被設計過
這一家最遠離惡意：沒有人要害誰。它留下的只是那個「有人坐下來、專門想辦法把它弄難」的感覺，所以語氣常常是抱怨裡帶著佩服。
adjective · C1
Extremely difficult or complicated, so hard that it feels as if someone made it that way on purpose; it is used about puzzles, exam questions, game levels and problems.
難得要命的