Source : Webster's Revised Unabridged Dictionary (1913)

Indubitably \In*du"bi*ta*bly\, adv.
   Undoubtedly; unquestionably; in a manner to remove all doubt.

         Oracles indubitably clear and infallibly certain.
                                                  --Barrow.

Source : WordNet®

indubitably
     adv : in a manner or to a degree that could not be doubted; "it
           was immediately and indubitably apparent that I had
           interrupted a scene of lovers"