The Collaborative International Dictionary of English v.0.48:
Doubtfully \Doubt"ful*ly\, adv. In a doubtful manner. [1913 Webster] Nor did the goddess doubtfully declare. --Dryden. [1913 Webster]WordNet (r) 3.0 (2006):
doubtfully adv 1: in a doubtful manner; "Gerald shook his head doubtfully" [syn: doubtfully, dubiously]