The Collaborative International Dictionary of English v.0.48:

Indubitably \In*du"bi*ta*bly\, adv.
   Undoubtedly; unquestionably; in a manner to remove all doubt.
   [1913 Webster]

         Oracles indubitably clear and infallibly certain.
                                                  --Barrow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indubitably
    adv 1: 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"; "his guilt was
           established beyond a shadow of a doubt" [syn:
           indubitably, beyond doubt, beyond a doubt, beyond
           a shadow of a doubt]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "indubitably":
   actually, admittedly, assuredly, beyond doubt, beyond question,
   by all odds, certainly, clearly, decidedly, demonstrably,
   dollars to doughnuts, doubtless, doubtlessly, for a certainty,
   for real, in all conscience, in all likelihood, in all probability,
   incontestably, indeed, indisputably, irrefragably, irrefutably,
   like enough, likely, manifestly, most likely, no doubt, noticeably,
   observably, obviously, out of question, patently, positively,
   presumably, presumptively, probably, quite, really, sensibly,
   seriously, ten to one, truly, unambiguously, undeniably,
   undoubtedly, unmistakably, unquestionably, verily, very like,
   very likely, visibly, without doubt