目瞪口呆

Origin and story

A related expression appears in the Tang collection Sanshui Xiaodu. In its supernatural tale, Xiahou Zhen makes an improper romantic appeal before a goddess's image. After returning, he becomes distracted and speechless; the narrative describes staring eyes and a tightly closed mouth. An apology and prayer are followed by recovery. This is a literary marvel, not a factual explanation of illness.

That wording is mu deng kou jin, rather than the modern kou dai. The exact idiom is attested in later fiction, including Romance of the Three Kingdoms and Dream of the Red Chamber, for someone so astonished or frightened that no answer comes.