In an exciting manner; in a way that causes excitement or enthusiasm.
From 'exciting' (present participle of 'excite') + '-ly' (adverb suffix). The '-ly' suffix comes from Old English 'lice' and turns adjectives into adverbs describing how something is done.
The adverb form 'excitingly' lets us describe how something happens—'The plot developed excitingly'—but it's less common than just saying 'exciting' as an adjective.
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