The Collaborative International Dictionary of English v.0.48:

Decidedly \De*cid"ed*ly\, adv.
   In a decided manner; indisputably; clearly; thoroughly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decidedly
    adv 1: without question and beyond doubt; "it was decidedly too
           expensive"; "she told him off in spades"; "by all odds
           they should win" [syn: decidedly, unquestionably,
           emphatically, definitely, in spades, by all odds]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "decidedly":
   absolutely, actually, affirmatively, and no mistake, assertively,
   assuredly, at all events, at any rate, bec et ongles, by all means,
   certainly, clearly, committedly, decisively, declaratively,
   definitely, demonstrably, determinedly, devotedly, distinctly,
   earnestly, emphatically, firmly, fixedly, for a certainty,
   for a fact, for certain, for real, for sure, forsooth,
   hammer and tongs, heart and soul, in all conscience,
   in all seriousness, in earnest, in good earnest, in truth, indeed,
   indeedy, indubitably, inflexibly, like a bulldog, like a leech,
   manifestly, most assuredly, most certainly, nothing else but,
   noticeably, observably, obstinately, obviously, of course,
   patently, perseveringly, persistently, positively, predicatively,
   purposefully, quite, really, relentlessly, resolutely, resolvedly,
   sensibly, seriously, sincerely, singlemindedly, staunchly,
   steadfastly, steadily, surely, tenaciously, to a certainty,
   tooth and nail, truly, unambiguously, undeniably, under oath,
   unequivocally, unmistakably, unswervingly, unyieldingly, verily,
   visibly, wholeheartedly, with a will, with constancy,
   with emphasis, with total dedication, without doubt