The Collaborative International Dictionary of English v.0.48:

dumbfound \dumbfound\ v.
   same as dumfound.

   Syn: confuse, perplex, throw, fox, befuddle, fuddle, bedevil,
        puzzle, mystify, baffle, bewilder, flummox, stupefy,
        stupify, nonplus, gravel, amaze, trounce, confound, be
        confusing to, make confused.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Dumfound \Dum"found`\, v. t. [imp. & p. p. Dumfounded; p. pr.
   & vb. n. Dumfounding.]
   To strike dumb; to confuse with astonishment. [Written also
   dumbfound.] --Spectator.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dumbfound
    v 1: be a mystery or bewildering to; "This beats me!"; "Got me--
         I don't know the answer!"; "a vexing problem"; "This
         question really stuck me" [syn: perplex, vex, stick,
         get, puzzle, mystify, baffle, beat, pose,
         bewilder, flummox, stupefy, nonplus, gravel,
         amaze, dumbfound]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "dumbfound":
   amaze, astonish, astound, awe, awestrike, bedaze, bedazzle,
   bewilder, boggle, bowl down, bowl over, choke, choke off, confound,
   confuse, daze, dazzle, dumbfounder, flabbergast, floor, gag, hush,
   hush-hush, muffle, muzzle, nonplus, overwhelm, paralyze, perplex,
   petrify, put to silence, quiet, quieten, shock, shush,
   shut down on, silence, soft-pedal, squash, squelch, stagger,
   startle, stifle, still, strike dead, strike dumb,
   strike with wonder, stun, stupefy, surprise, throttle