In a wickedly clever or malicious manner; with demonic cunning or cruel delight; in a way that is extremely difficult or troublesome.
Adverbial form of 'fiendish' (fiend + '-ish' suffix, meaning having the quality of, plus '-ly'). The '-ish' suffix appears in Middle English, likely influenced by Old Norse '-iskr,' and creates adjectives from nouns meaning 'like' or 'having the character of.'
Modern English uses 'fiendishly' playfully in contexts like 'fiendishly difficult puzzle,' which dilutes the word's demonic intensity—this linguistic softening happens when vivid words get used for mild situations repeatedly.
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