The Collaborative International Dictionary of English v.0.48:

Presumably \Pre*sum"a*bly\, adv.
   In a presumable manner; by, or according to, presumption.
   [1913 Webster]

WordNet (r) 3.0 (2006):

presumably
    adv 1: by reasonable assumption; "presumably, he missed the
           train" [syn: presumably, presumptively]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "presumably":
   all things considered, as it were, assumably, assumedly,
   assumptively, by all odds, certainly, dollars to doughnuts,
   doubtless, doubtlessly, in all likelihood, in all probability,
   in seeming, indubitably, like enough, likely, most likely,
   no doubt, presumedly, presumptively, probably, quasi, reputedly,
   seemingly, supposably, supposedly, suppositionally,
   supposititiously, surely, ten to one, undoubtedly, unquestionably,
   very like, very likely