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.