'devilishly' originates from English, specifically from the adjective 'devilish' (formed from 'devil' + suffix '-ish') with the adverbial suffix '-ly'.

'devilish' changed from Middle English 'devilish' (from 'devil' + '-ish') and became 'devilishly' when the Old English adverbial suffix '-ly' (from 'lic'/'lice') was added in later Middle English.

Initially it meant 'of or like the devil' (i.e., possessing devil-like qualities), but over time it also developed an intensifying meaning 'extremely' used in phrases like 'devilishly difficult'.