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®

dumbfound
     v : be a mystery or bewildering to; "This beats me!"; "Got me--I
         don't know the answer!"; "a vexing problem"; "This
         question really stuck me" [syn: {perplex}, {vex}, {stick},
          {get}, {puzzle}, {mystify}, {baffle}, {beat}, {pose}, {bewilder},
          {flummox}, {stupefy}, {nonplus}, {gravel}, {amaze}]