If you are dumbfounded, you are so surprised that you cannot think or speak for a beat. The face is often still; the mouth may be open. It is a feeling word, a bit formal or story-like, but people do say it in real life when the surprise is huge — a gift, a score, bad news, something that should not be possible. It is the usual everyday form of this family: we say I was dumbfounded far more than anyone dumbfounded me. Though actually it is not quite the same as scared. You can be dumbfounded by something wonderful. And it is not ‘feeling stupid’, even though dumb sits inside it; the old piece means you have gone wordless.