In a dandyish manner; in a way that is excessively concerned with fashion and affected elegance.
From 'dandyish' plus the adverb-forming suffix '-ly' (from Old English, originally meaning 'like' or 'similar to'). This creates the adverbial form of the adjective.
English's productivity with suffixes means you can take almost any adjective and add '-ly' to turn it into an adverb—this creates infinite possibilities, which is why English speakers can describe almost anything in motion or manner.
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