Source : Webster's Revised Unabridged Dictionary (1913)

Doubtfully \Doubt"ful*ly\, adv.
   In a doubtful manner.

         Nor did the goddess doubtfully declare.  --Dryden.

Source : WordNet®

doubtfully
     adv : in a doubtful manner; "Gerald shook his head doubtfully"
           [syn: {dubiously}]