In a distended manner; in a way that shows stretching or swelling.
From distended + -ly (adverb suffix). Distended comes from Latin distendere (to stretch apart), so this describes the manner of stretching.
Adverbs made from past participles like this one are less common than regular adverbs—we usually say 'with a distended appearance' rather than 'distendedly,' which shows how English sometimes prefers paraphrasing over awkward word formations.
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