What does fiendish mean?
Fiendish is an adjective with two closely related senses. In its older, literal sense it describes someone or something cruel, wicked, or diabolical — behaviour worthy of a fiend, an old word for a devil or evil spirit. Calling a villain's smile or an act of torture fiendish stresses its almost supernatural malice. In modern everyday use, however, the word more often means extremely difficult, complicated, or cunningly clever: a fiendish puzzle, a fiendish exam question, or a fiendishly intricate piece of machinery. This second sense usually carries a note of exasperated admiration rather than real moral judgement, which gives the word a slightly playful register despite its sinister origins. Derived forms include the adverb fiendishly and the noun fiendishness. Because it vividly links difficulty with devilry, fiendish remains a favourite of journalists, setters of crosswords, and anyone describing a challenge that seems almost deliberately cruel.
adjectiveExtremely cruel, wicked, or devilish in nature; also, extremely difficult, complicated, or ingenious, as in a fiendish puzzle or fiendishly hard problem.
- Devilishly cruel or wicked; characteristic of a fiend or evil spirit.
- Extremely difficult, complex, or cunningly ingenious (often describing puzzles, problems, plans, or machines).
"The final crossword clue was so fiendish that even the seasoned solvers gave up on it."
"The villain revealed a fiendish scheme to steal the museum's crown jewels."
"She finally cracked the fiendish logic puzzle after three days of effort."
Fiendish literally means 'like a fiend' — and since a fiend is a devil, calling a maths exam fiendish is quietly accusing it of demonic cruelty.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of fiendish
Fiendish is formed from fiend — an Old English word (fēond) meaning enemy or devil, originally the present participle of a verb meaning 'to hate' — combined with the adjectival suffix -ish. It has been used in English since roughly the sixteenth century to mean devil-like or wicked, and its extended sense of 'extremely difficult' developed later from the idea of diabolical cunning. Related words include fiend itself and the compounds fiendish and fiendlike; the root fēond also survives in the ordinary modern word friend's ancient opposite, foe, which comes from the same Germanic verb of hatred.
Related word forms
How fiendish is actually used
Fiendish is common in both British and American English and carries a slightly playful, literary flavour even when negative. The 'extremely difficult' sense is frequent in informal contexts, especially British usage (e.g. 'a fiendishly tricky test'), and often carries a tone of grudging admiration rather than genuine moral condemnation.
Easily confused with fiendish
"Finish" means to bring something to an end or complete it, while "fiendish" describes something wicked or extremely difficult; the two merely sound similar.