Source : Webster's Revised Unabridged Dictionary (1913)

Astonish \As*ton"ish\, v. t. [imp. & p. p. {Astonished}; p. pr.
   & vb. n. {Astonishing}.] [OE. astonien, astunian, astonen,
   OF. estoner, F. ['e]tonner, fr. L. ex out + tonare to
   thunder, but perhaps influenced by E. stun. See {Thunder},
   {Astound}, {Astony}.]
   1. To stun; to render senseless, as by a blow. [Obs.]

            Enough, captain; you have astonished him. [Fluellen
            had struck Pistol].                   --Shak.

            The very cramp-fish [i. e., torpedo] . . . being
            herself not benumbed, is able to astonish others.
                                                  --Holland.

   2. To strike with sudden fear, terror, or wonder; to amaze;
      to surprise greatly, as with something unaccountable; to
      confound with some sudden emotion or passion.

            Musidorus . . . had his wits astonished with sorrow.
                                                  --Sidney.

            I, Daniel . . . was astonished at the vision. --Dan.
                                                  viii. 27.

   Syn: To amaze; astound; overwhelm; surprise.

   Usage: {Astonished}, {Surprised}. We are surprised at what is
          unexpected. We are astonished at what is above or
          beyond our comprehension. We are taken by surprise. We
          are struck with astonishment. --C. J. Smith. See
          {Amaze}.

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®

astonishing
     adj 1: surprising greatly; "she does an amazing amount of work";
            "the dog was capable of astonishing tricks" [syn: {amazing}]
     2: so surprisingly impressive as to stun or overwhelm; "such an
        enormous response was astonishing"; "an astounding
        achievement"; "the amount of money required was
        staggering"; "suffered a staggering defeat"; "the figure
        inside the boucle dress was stupefying" [syn: {astounding},
         {staggering}, {stupefying}]