The Collaborative International Dictionary of English v.0.48:

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.]
      [1913 Webster]

            Enough, captain; you have astonished him. [Fluellen
            had struck Pistol].                   --Shak.
      [1913 Webster]

            The very cramp-fish [i. e., torpedo] . . . being
            herself not benumbed, is able to astonish others.
                                                  --Holland.
      [1913 Webster]

   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.
      [1913 Webster]

            Musidorus . . . had his wits astonished with sorrow.
                                                  --Sidney.
      [1913 Webster]

            I, Daniel . . . was astonished at the vision. --Dan.
                                                  viii. 27.
      [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

astonish
    v 1: affect with wonder; "Your ability to speak six languages
         amazes me!" [syn: amaze, astonish, astound]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "astonish":
   affright, alarm, amaze, ambush, astound, awe, awestrike, bedaze,
   bedazzle, bewilder, boggle, bowl down, bowl over, bushwhack,
   catch off-guard, catch unawares, come from behind,
   come upon unexpectedly, confound, daze, dazzle, do the unexpected,
   drop in on, dumbfound, dumbfounder, flabbergast, floor, overwhelm,
   paralyze, perplex, petrify, pounce upon, pull up short, shock,
   spring a surprise, spring upon, stagger, startle, strike dead,
   strike dumb, strike with wonder, stun, stupefy, surprise,
   take by surprise, take short, take unawares, terrify