Having the characteristics, style, or manner of an author; befitting or relating to a writer.
From 'author' plus the adjective-forming suffix '-ly' (from Old English '-lic'). This creates an adjective meaning 'in the manner of an author.'
English lets us turn almost any noun into an adjective with '-ly'—'kingly,' 'brotherly,' 'scholarly'—but 'authorly' is relatively rare, showing that some combinations feel more natural than others depending on what writers actually use.
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