In a neat, trim, and fashionable manner; with a smart and stylish appearance or bearing.
From 'dapper' plus the adverbial suffix '-ly'; follows standard English patterns for converting adjectives to adverbs.
Dapperly is one of those wonderfully specific adverbs English can build—it doesn't just mean 'neatly' but conveys a whole attitude of smart confidence, the way someone who is dapperly dressed carries themselves.
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