In a bestial manner; in a savage, cruel, brutal, or animal-like way.
From bestial + -ly (adverb-forming suffix). Bestial comes from Latin bestia (beast). The -ly ending transforms adjectives into adverbs, a distinctly English feature.
Old English used -lice for adverbs, which became -ly, and it's one of the most productive suffixes in English—you can add it to almost any adjective and people will understand it, making English adverb-creation remarkably flexible compared to Romance languages.
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