Source : WordNet®

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}]
            [ant: {believably}, {credibly}]
     2: in an unbelievable manner; "he was unbelievably angry" [ant:
         {believably}]