From WordNet (r) 3.0 (2006) [wn]:
unbelievably
adv 1: not easy to believe; "behind you the coastal hills plunge
to the incredibly blue sea backed by the Turkish
mountains" [syn: incredibly, improbably,
implausibly, unbelievably] [ant: believably,
credibly, plausibly, probably]
2: in an unbelievable manner; "he was unbelievably angry" [ant:
believably]