In a manner characteristic of or befitting an adult; with maturity and adult-like behavior.
From 'adult' + '-ly' adverbial suffix. Relatively rare in modern English, used to describe behavior or actions done in a mature way.
While 'adultly' is grammatically correct, English speakers much prefer 'maturely,' 'as an adult,' or 'in an adult manner.' This shows how language naturally gravitates toward clearer or more frequent expressions over technically correct alternatives.
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