Devilishly is from devilish + -ly. Devil is from Old English deofol, from Latin diabolus, from Ancient Greek diabolos ('slanderer'), from diaballein ('to slander, throw across'). The ancestry passes through Old English deofol and Ancient Greek diabolos. The suffix -ly forms an adverb or adjective from the base, so the English form is shaped by recognizable word formation as well as by its inherited or borrowed base. In present use as an adverb, it means "in a mischievous or fiendish way; or extremely", keeping the historical formation close to the word's ordinary semantic work. Related words include devilish, devil, and diabolical.