The Collaborative International Dictionary of English v.0.48:

Astonishing \As*ton"ish*ing\, a.
   Very wonderful; of a nature to excite astonishment; as, an
   astonishing event.
   [1913 Webster]

   Syn: Amazing; surprising; wonderful; marvelous.
        [1913 Webster] As*ton"ish*ing*ly, adv. --
        As*ton"ish*ing*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

astonishingly
    adv 1: in an amazing manner; to everyone's surprise; "amazingly,
           he finished medical school in three years" [syn:
           amazingly, surprisingly, astonishingly]