In a frowning manner; while frowning or with a frown on one's face.
From 'frown' plus the present participle suffix '-ing', then the adverbial suffix '-ly', which converts adjectives and participles into adverbs describing how an action is performed.
Adverbs ending in '-ingly' are beautiful because they add a visual element to an action—'frowningly' makes us see the expression on someone's face while they do something, not just what they do.
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