Source : Webster's Revised Unabridged Dictionary (1913)

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

   Syn: Amazing; surprising; wonderful; marvelous.
        {As*ton"ish*ing*ly}, adv. -- {As*ton"ish*ing*ness}, n.

Source : WordNet®

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