Fiendish means extremely cruel, or extremely clever and difficult. Fiendish cunning, a fiendish puzzle, fiendishly difficult.
The second sense is the commoner and is nearly admiring, since a fiendish crossword is a compliment to its setter. The fiend behind it is the Devil, and the cruel sense keeps that weight.
1adjectivefiendishExtremely cruel, or extremely clever and difficult.
“The villain came up with a fiendish plan to trap the hero in a burning tower.”
fiendish is a quality.
Proto-Indo-European
*peh₁-
Proto-Germanic
*fijandz
Proto-West Germanic
*fijand
Old English
fēond — enemy
Middle English
fend
English
fiend
English, today
fiendish
Built from
From fiend + -ish. fiend comes from Middle English fend, feend (“enemy; demon”), from Old English fēond (“enemy”), Proto-West Germanic fijand, from Proto-Germanic fijandz. 2 Cognate with Scots fient (“fiend”), stq Fäind (“enemy, fiend, foe”), cim faint (“enemy, fiend”), Dutch vijand (“enemy”), German Feind (“enemy, fiend, foe”), wym faeind, fajnd (“enemy”), Yiddish פֿײַנד, פֿײַנט (“enemy”), Danish fjende (“adversary, enemy, foe”), Icelandic fjandi (“enemy; fiend, demon, devil”), Norwegian Bokmål -, Norwegian Nynorsk - and Swedish fiende (“enemy”), Old Norse fjándi (“enemy; devil”), Gothic 𐍆𐌹𐌰𐌽𐌳𐍃, 𐍆𐌹𐌾𐌰𐌽𐌳𐍃 (“enemy, foe”). The Old Norse and Gothic terms are present participles of the corresponding verbs fjá/𐍆𐌹𐌾𐌰𐌽 (“to hate”), from Proto-Indo-European peh₁- (“to hate”) (compare Sanskrit पीयति (“(he) reviles”)).
Etymology from Wiktionary, CC BY-SA