In a sparkling, shining manner; with a bright and dazzling quality.
From glittering (present participle of glitter) + -ly (adverb-forming suffix). The adverb form allows the adjective to modify verbs and other adjectives.
Adding -ly to adjectives is English's favorite trick for creating adverbs, but 'glitteringly' has a musical quality—say it aloud and the 'gl' and 'tt' sounds create a cascade effect that mirrors the meaning itself.
Complete word intelligence in one call. Free tier — 50 lookups/day.