The Collaborative International Dictionary of English v.0.48:

dumbfounding \dumbfounding\ adj.
   causing astonishment. [Narrower terms: incredible (vs.
   credible), unbelievable]

   Syn: astonishing, astounding, dumfounding.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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