The Collaborative International Dictionary of English v.0.48:

Certainly \Cer"tain*ly\, adv.
   Without doubt or question; unquestionably.
   [1913 Webster]

WordNet (r) 3.0 (2006):

certainly
    adv 1: definitely or positively (`sure' is sometimes used
           informally for `surely'); "the results are surely
           encouraging"; "she certainly is a hard worker"; "it's
           going to be a good day for sure"; "they are coming, for
           certain"; "they thought he had been killed sure enough";
           "he'll win sure as shooting"; "they sure smell good";
           "sure he'll come" [syn: surely, certainly, sure,
           for sure, for certain, sure enough, sure as
           shooting]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "certainly":
   OK, Roger, absolutely, actually, all right, alright, alrighty,
   amen, and no mistake, as you say, assuredly, at all events,
   at any rate, aye, by all means, clearly, da, decidedly, decisively,
   definitely, demonstrably, distinctly, exactly, factually,
   fatefully, fine, for a certainty, for a fact, for certain,
   for real, for sure, forsooth, good, good enough, hear,
   historically, in all conscience, in fact, in reality, in truth,
   in very sooth, indeed, indeedy, indefeasibly, indubitably,
   ineluctably, inescapably, inevasibly, inevitably, inexorably,
   inflexibly, irrevocably, ja, just so, mais oui, manifestly,
   most assuredly, most certainly, naturally, naturellement,
   necessarily, nothing else but, noticeably, observably, obviously,
   of a truth, of course, okay, oui, patently, positively, precisely,
   quite, rather, really, really-truly, relentlessly, right, righto,
   sensibly, seriously, sure, sure thing, surely, to a certainty,
   to be sure, truly, unambiguously, unavoidably, uncontrollably,
   undeniably, undoubtedly, unequivocally, unmistakably,
   unpreventably, unyieldingly, verily, veritably, very well, visibly,
   well and good, why yes, with truth, without doubt, yea, yeah, yep,
   yes, yes indeed, yes indeedy, yes sir, yes sirree