The Collaborative International Dictionary of English v.0.48:

Doubtless \Doubt"less\, a.
   Free from fear or suspicion. [Obs.]
   [1913 Webster]

         Pretty child, sleep doubtless and secure. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Doubtless \Doubt"less\, adv.
   Undoubtedly; without doubt.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "doubtless":
   absolutely, admittedly, assumably, assured, believing,
   beyond doubt, beyond question, by all odds, certain, certainly,
   confident, convinced, definitely, devout, dogmatic,
   dollars to doughnuts, doubtlessly, faithful, fideistic,
   for certain, for sure, impressed with, in all likelihood,
   in all probability, incontestably, indisputably, indubious,
   indubitably, irrefragably, irrefutably, like enough, likely,
   make no mistake, most likely, naturally, no doubt, out of question,
   past dispute, persuaded, pietistic, pious, pistic, positive,
   positively, presumably, presumptively, probably, questionless,
   satisfied, secure, sold on, supposedly, sure, surely, ten to one,
   truly, unchallenged, uncontested, uncontradicted, uncontroversial,
   uncontroverted, undeniably, under the impression, undisputed,
   undoubted, undoubtedly, undoubtful, undoubting, unequivocally,
   unquestionably, unquestioned, very like, very likely,
   without doubt