The Collaborative International Dictionary of English v.0.48:

Surely \Sure"ly\, adv.
   1. In a sure or certain manner; certainly; infallibly;
      undoubtedly; assuredly.
      [1913 Webster]

            In the day that thou eatest thereof thou shalt
            surely die.                           --Gen. ii. 17.
      [1913 Webster]

            He that created something out of nothing, surely can
            raise great things out of small.      --South.
      [1913 Webster]

   2. Without danger; firmly; steadly; securely.
      [1913 Webster]

            He that walketh uprightly walketh surely. --Prov. x.
                                                  9.
      [1913 Webster]

WordNet (r) 3.0 (2006):

surely
    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:

100 Moby Thesaurus words for "surely":
   OK, Roger, absolutely, all right, alright, alrighty, amen,
   and no mistake, as you say, assuredly, at all events, at any rate,
   aye, beyond question, by all means, certainly, clearly,
   confidently, da, decidedly, decisively, definitely, determinedly,
   distinctly, doggedly, doubtless, doubtlessly, exactly, fatefully,
   fine, firmly, for a certainty, for a fact, for certain, for sure,
   forsooth, good, good enough, hear, in truth, indeed, indeedy,
   indefeasibly, indubitably, ineluctably, inescapably, inevasibly,
   inevitably, inexorably, inflexibly, irrevocably, ja, just so,
   mais oui, most assuredly, most certainly, naturally, naturellement,
   necessarily, nothing else but, of course, okay, oui, positively,
   precisely, quite, rather, really, relentlessly, right, righto,
   securely, solidly, steadily, sure, sure thing, to a certainty,
   to be sure, truly, unavoidably, uncontrollably, undoubtedly,
   unequivocally, unfalteringly, unmistakably, unpreventably,
   unquestionably, unswervingly, unyieldingly, very well,
   well and good, why yes, yea, yeah, yep, yes, yes indeed,
   yes indeedy, yes sir, yes sirree