Added 10 October 2026 fiendishly /ˈfiːndɪʃli/ adverb Categories: Describing WordsRare Words adverb ! Definition from WordNet 3.1, Princeton University. Pronunciation from Wiktionary, via Kaikki.org. More about fiendishly at Wordnik → Where our words come from → As a devil; in an evil manner. his writing could be diabolically satiric Related Words: diabolicallydevilishlyabominablyappallinglybrutallyterriblywildlysavagelyseverelyruthlesslyfiendfullywickedly Back to today's word