In a manner involving or characterized by choosing; while choosing or with the action of choosing.
From 'choose' with the gerund/present participle suffix '-ing' plus the adverb suffix '-ly,' creating an adverb from a verb.
Adverbs ending in '-ingly' are rare and often sound awkward because English prefers simpler adverbs—you'd more naturally say 'by choosing' than 'choosingly,' showing that not all grammatically possible words actually survive in real language!
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