In a way that expresses doubt, uncertainty, or lack of confidence.
Formed from 'doubtful' (adjective, from 'doubt' plus '-ful,' meaning 'full of') plus the adverbial suffix '-ly.' This is the standard modern way to create an adverb from the adjective 'doubtful,' following regular English patterns.
When someone responds 'doubtfully' in dialogue, it's often more powerful than saying 'uncertainly' because readers immediately picture the facial expression—the furrowed brow and head tilt that comes with 'I'm not sure about that.'
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