Source : Webster's Revised Unabridged Dictionary (1913)

Masterfully \Mas"ter*ful*ly\, adv.
   In a masterful manner; imperiously.

         A lawless and rebellious man who held lands masterfully
         and in high contempt of the royal authority.
                                                  --Macaulay.

Source : WordNet®

masterfully
     adv 1: in a skillfully masterful manner; with the skill of a
            master; "the young boy played the violin sonata
            masterfully"
     2: in a domineering and masterful manner; "his gaze held hers
        masterfully"