In a chesty manner; in a way that is bold, self-assertive, or showing off one's chest or confidence.
From 'chesty' (adjective) with the adverb-forming suffix '-ly,' following standard English patterns for converting adjectives to adverbs that describe how something is done.
Adverbs ending in '-ily' are almost always formed from adjectives ending in '-y,' and 'chestily' is a perfect example—it takes the attitude of 'chesty' and turns it into a way of acting.
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