'fiendishly' originates from English, specifically the word 'fiendish' + the adverbial suffix '-ly', where 'fiend' (Old English 'feond') meant 'enemy' or 'devil' and the suffix '-ly' meant 'in the manner of'.

'fiendish' changed from Middle English forms such as 'fiendisch' (from Old English 'feond' + the adjectival suffix '-isc'), and adding the adverbial suffix '-ly' produced the modern English 'fiendishly'.

Initially it meant 'of or like a fiend (enemy/devil)'; over time it also developed a secondary use as an intensifier meaning 'extremely' or 'very'.