In a dressy or formal manner; fashionably or elegantly with respect to clothing.
From 'dressy' (stylish or formal) with the adverbial suffix '-ly,' which comes from Old English 'lich' (body or form).
Adding '-ly' to create adverbs is one of English's most productive word-making tools—you can describe almost any action as being done 'dressily' or any other adjective-ly, even if the combination is unusual or invented.
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