The Collaborative International Dictionary of English v.0.48:

dumfounding \dumfounding\ adj.
   same as dumbfounding.

   Syn: astonishing, astounding, dumbfounding.
        [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):

dumfounding
    adj 1: bewildering or striking dumb with wonder [syn:
           astounding, dumbfounding, dumfounding]