In a manner showing contempt or scorn; with an air of superiority and disrespect.
From disdainful + -ly (adverb suffix). Disdainful comes from disdain + -ful (full of), creating the standard pathway to the adverb form.
Authors use 'disdainfully' in dialogue tags to tell us exactly how someone said something—'he replied disdainfully'—and it instantly makes that character feel arrogant, making it one of the most powerful characterization adverbs.
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