If you are dumbstruck, you are so shocked or surprised that you cannot speak. The picture is physical: something has ‘hit’ you, and the words will not come. It is stronger and more sudden than ordinary surprise, and a little more literary than just shocked. People use it for a crash, a reveal, a piece of news that stops the room. You can be happily amazed and still dumbstruck, but the word leans toward being stunned, even frozen. Though actually dumb here is again the old ‘cannot speak’, the same idea as strike someone dumb — not ‘stupid’. Struck is the past of strike, as if the feeling hit you.

嚇得說不出話、愣住