In a manner relating to or characteristic of cannibalism; in a way that involves eating human flesh or resembles such behavior.
From 'cannibalistic' plus the adverbial suffix '-ly,' creating the adverbial form to describe actions or behaviors done in a cannibalistic way.
You can modify almost any English adjective with '-ly' to make an adverb, so 'cannibalistically' is grammatically perfect but rarely used—we usually just say 'like a cannibal'!
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