In a glumpy manner; in a way that is sulky, morose, or characterized by heavy gloominess.
From 'glumpy' + '-ly' (adverb suffix). Following standard English adverbial construction patterns.
The adverb 'glumpily' describes not just sadness but a kind of irritable, slouching melancholy—the teenager slumping into a chair 'glumpily' rather than expressing emotions directly.
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