The verb dumbfound means to puzzle, mystify, or amaze. If people never expected you to amount to much in high school, but you grew up to be a rocket scientist, you will surely dumbfound your former classmates at your next reunion.
The word dumbfound is a combination of the words dumb and confound. Dumb, in the original sense, means unable to speak. Confound is from the Latin word confundere, which means to mix together as well as to confuse. Thus the blended word dumbfound has the sense of to confuse to the point of speechlessness. If you see a solar eclipse for the first time, it might dumbfound you.