In a beastly or savage manner; in a way that resembles or displays beast-like behavior.
From 'beastly' (adjective) plus the adverbial suffix '-ly' (Old English '-lice'), a standard transformation that turns descriptive words into manner adverbs.
The '-ly' adverbial suffix comes from Old English 'lice' meaning 'like' or 'in the manner of'—so 'beastlily' literally means 'in a beast-like manner,' making it a linguistic fossil of how adverbs evolved.
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