From WordNet (r) 2.0 (August 2003) [wn]:
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: 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] [ant: incredibly]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]