The characters combine to create the literal, visceral image: “mutually moistening with saliva.” This striking visual metaphor perfectly captures the desperation and selfless dedication inherent in the idiom's meaning.
The origin of 相濡以沫 comes from the classic Daoist text, the `Zhuangzi` (《庄子》). The full parable is: “When the spring dries up, the fish are stranded together on the land. They moisten each other with their breath and wet each other with their spit, but it would be better to forget each other in the rivers and lakes (相濡以沫，不如相忘于江湖).” Interestingly, Zhuangzi's original point was a Daoist critique. He argued that while the fish helping each other is a noble struggle, it's a compromised existence. The ideal state is for the fish to be swimming freely in the vast “rivers and lakes” (江湖, jiānghú), so vast they don't even need to be aware of each other. This represents the Daoist ideal of natural, unconstrained freedom. However, in modern Chinese culture, this second part is almost always dropped. The idiom is celebrated for the first part alone: the beautiful, romantic, and selfless act of mutual support. It has come to represent the ultimate relationship goal—a bond so strong it can withstand any hardship. Comparison to Western Concepts: This idiom is often translated as “through thick and thin” or “for better or for worse.” However, there's a key difference. The Western phrases are typically a promise about the future. 相濡以沫 is almost always a description of a shared past. It implies that the “worse” or “thin” has already happened, and the relationship not only survived but was strengthened because of it. It carries a much stronger connotation of having overcome dire circumstances, not just everyday ups and downs.
This is a formal, literary, and highly respected idiom. It's not used in casual, everyday conversation.
The connotation is overwhelmingly positive and carries a sense of deep respect and admiration.