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]