Dumbfound is defined as to amaze someone, or to make someone speechless because of shock.(verb)
If you say something so smart no one can believe it came out of your mouth and everyone is amazed, this is an example of when you dumbfound them.
See dumbfound in Webster's New World College Dictionary
Origin: dumb + (con)found
See dumbfound in American Heritage Dictionary 4
also dum·foundtransitive verb dumb·found·ed also dum·found·ed, dumb·found·ing also dum·found·ing, dumb·founds also dum·founds
Origin: + (con)found.
Learn more about dumbfound