These characters combine literally to mean “[even] water cannot leak through.” This powerful, almost physical image is used metaphorically to describe any space that is packed to its absolute limit, leaving no gaps or pathways.
The idiom 水泄不通 is deeply rooted in the physical reality of life in China, a country with a vast population. The experience of being in an overwhelmingly dense crowd is common, whether during the Spring Festival travel rush (春运 chūnyùn), on a major shopping street, or at a popular tourist site during a national holiday. The idiom provides a shared, instantly understood image for this collective experience. Compared to an English phrase like “packed like sardines,” which focuses on the individuals being crammed together, 水泄不通 emphasizes the impenetrability of the collective mass. The focus is less on the discomfort of the individual and more on the objective state of the entire space being sealed shut by the sheer volume of people. It reflects a perspective accustomed to viewing and navigating large-scale collective situations.
水泄不通 is a common and versatile idiom used in both spoken and written Chinese. It's appropriate for everything from casual conversation to formal news reports.