The Collaborative International Dictionary of English v.0.48:

Undoubted \Un*doubt"ed\, a.
   Not doubted; not called in question; indubitable;
   indisputable; as, undoubted proof; undoubted hero. --
   Un*doubt"ed*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undoubtedly
    adv 1: without doubt; certainly; "it's undoubtedly very
           beautiful" [syn: undoubtedly, doubtless,
           doubtlessly]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "undoubtedly":
   actually, admittedly, assuredly, beyond doubt, beyond question,
   certainly, clearly, definitely, doubtless, doubtlessly, easily,
   explicitly, factually, forsooth, historically, in fact, in reality,
   in truth, in very sooth, incontestably, indeed, indeedy,
   indisputably, indubitably, irrefragably, irrefutably, no doubt,
   nothing else but, obviously, of a truth, out of question, really,
   really-truly, surely, truly, undeniably, unmistakably,
   unquestionably, verily, veritably, with truth, without doubt