In a wicked, cruel, or morally wrong manner.
From Old English 'yfel' meaning bad or wicked, plus the adverbial suffix '-ly' added in Middle English to create an adverb form. The meaning has remained consistent throughout English history.
The word 'evil' is one of the oldest moral concepts in English, but 'evilly' is surprisingly rare in modern writing because we usually just say 'wickedly' or 'cruelly'—it sounds almost comically old-fashioned when you use it.
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