In a manner that glitters or sparkles brightly; with a bright, shining quality.
From glistering + adverbial suffix '-ly'. The rare formation shows how English can build adverbs from participles for poetic effect.
This is such a rare word that it might appear in historical poetry or fantasy novels—imagine 'the dragon's eyes glisteringly bright'—it's the kind of word that makes readers pause and savor the sound!
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