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]