The Collaborative International Dictionary of English v.0.48:

Flabbergast \Flab"ber*gast\ (fl[a^]b"b[~e]r*g[a^]st), v. t. [Cf.
   Flap, and Aghast.]
   To astonish; to strike with wonder, esp. by extraordinary
   statements. [Jocular] --Beaconsfield.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flabbergast
    v 1: overcome with amazement; "This boggles the mind!" [syn:
         flabbergast, boggle, bowl over]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "flabbergast":
   amaze, astonish, astound, awe, awestrike, bedaze, bedazzle,
   bewilder, boggle, bowl down, bowl over, confound, daze, dazzle,
   dumbfound, dumbfounder, overwhelm, paralyze, perplex, petrify,
   shock, stagger, startle, strike dead, strike dumb,
   strike with wonder, stun, stupefy, surprise