In an astonished manner; with the manner or feeling of being greatly surprised or amazed.
From 'astonished' (past participle of 'astonish') + '-ly' (forming adverbs). A rare but grammatically valid adverbial form, though 'astonishingly' is more common.
English speakers almost never use 'astonishedly' because 'astonishingly' flows better—but grammatically, nothing's wrong with it, showing how English has many valid paths that some speakers just avoid.
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