Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

dumfounded
     adj : as if struck dumb with astonishment and surprise; "a circle
           of policement stood dumbfounded by her denial of having
           seen the accident"; "the flabbergasted aldermen were
           speechless"; "was thunderstruck by the news of his
           promotion" [syn: {dumbfounded}, {flabbergasted}, {stupefied},
            {thunderstruck}]