These four characters combine to create a powerful and literal image: “Eyes staring, mouth blank.” It's a snapshot of a person completely overwhelmed by what they are witnessing.
As a `chengyu` (成语), 目瞪口呆 is part of a rich tradition of four-character idioms that add sophistication and imagery to the Chinese language. Its power lies in its universality and descriptive nature. Unlike some chengyu rooted in obscure historical anecdotes, the meaning of 目瞪口呆 is immediately clear from its component characters. In Western culture, we might say someone was “gobsmacked,” “flabbergasted,” or that a situation was “jaw-dropping.” These are excellent parallels. However, 目瞪口呆 is subtly different as it focuses entirely on the physical reaction of the stunned person. “Jaw-dropping” describes the event, but “mùdèngkǒudāi” describes the person experiencing the event. It’s a very visual and almost cinematic way to describe a state of shock, frequently used in storytelling, news reporting, and everyday conversation to add dramatic flair.
目瞪口呆 is common in both spoken and written Chinese. It's used to express a strong degree of surprise, far beyond simple astonishment.