Wordnet 3.0
ADVERB (2)
1. 
 not easy to believe; 
- Example: "behind you the coastal hills plunge to the incredibly blue sea backed by the Turkish mountains"[syn: incredibly, improbably, implausibly, unbelievably]
2.  in an unbelievable manner; 
- Example: "he was unbelievably angry"
WordNet (r) 3.0 (2006):
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]