The Collaborative International Dictionary of English v.0.48:

Doubtlessly \Doubt"less*ly\, adv.
   Unquestionably. --Beau. & Fl.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "doubtlessly":
   absolutely, admittedly, beyond doubt, beyond question, by all odds,
   definitely, dollars to doughnuts, doubtless, easily,
   in all likelihood, in all probability, incontestably, indeed,
   indisputably, indubitably, irrefragably, irrefutably, like enough,
   likely, most likely, no doubt, out of question, positively,
   presumably, presumptively, probably, really, ten to one, truly,
   undeniably, undoubtedly, unequivocally, unquestionably, very like,
   very likely, without doubt