In a manner that is brown in color or characteristic of brown.
From 'brown' (Old English 'brun') plus the adverbial suffix '-ly', which converts adjectives into adverbs describing how something occurs or appears.
While 'brownly' is rarely used in modern English, it represents how English speakers have historically tried to describe actions or appearances related to colors—a linguistic impulse that mostly died out, replaced by phrases like 'in a brown manner' or simply using 'brown' as an adjective.
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