The Collaborative International Dictionary of English v.0.48:

Probably \Prob"a*bly\, adv.
   In a probable manner; in likelihood.
   [1913 Webster]

         Distinguish between what may possibly and what will
         probably be done.                        --L'Estrange.
   [1913 Webster]

WordNet (r) 3.0 (2006):

probably
    adv 1: with considerable certainty; without much doubt; "He is
           probably out of the country"; "in all likelihood we are
           headed for war" [syn: probably, likely, in all
           likelihood, in all probability, belike]
    2: easy to believe on the basis of available evidence; "he
       talked plausibly before the committee"; "he will probably win
       the election" [syn: credibly, believably, plausibly,
       probably] [ant: implausibly, improbably, incredibly,
       unbelievably]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "probably":
   after a time, after a while, afterward, afterwards,
   all things considered, anon, assumably, before long, by all odds,
   by and by, by destiny, dollars to doughnuts, doubtless,
   doubtlessly, fatally, hopefully, imminently, in aftertime,
   in all likelihood, in all probability, in the future, indubitably,
   later, like enough, likely, most likely, no doubt, perhaps,
   possibly, predictably, presumably, presumptively, proximo, quite,
   soon, ten to one, tomorrow, undoubtedly, unquestionably, very like,
   very likely