The term literally translates to “a beautiful-woman soulmate.” It combines the appreciation for her female identity (`红颜`) with the profound depth of the non-romantic, intellectual bond (`知己`).
The concept of `红颜知己` is deeply rooted in China's literary and social history. In a traditional society with stricter separations between genders, the idea of a pure, intellectual friendship between a man and a woman was a rare and highly idealized notion. It represented a connection that transcended societal norms and physical desire, focusing instead on a meeting of minds. A helpful Western comparison might be the concept of a “platonic soulmate” or a “work wife,” but with crucial differences. Western concepts tend to de-emphasize gender, focusing solely on the platonic bond. In contrast, `红颜知己` explicitly highlights the person's gender (`红颜` - beautiful woman). This acknowledgment adds a layer of complexity, beauty, and subtle tension. It recognizes and values the female perspective and energy in the friendship, making the bond distinct from a man's friendship with his male `哥们儿 (gēmenr)` (buddies). This term speaks to the cultural value placed on deep, fated relationships (`缘分 yuánfèn`) and the appreciation for nuanced emotional connections that don't fit into simple categories like “friend” or “lover.”
In modern China, `红颜知己` is still used, but its meaning can be a bit of a “gray area.”
Its usage is generally semi-formal or literary. You would not use it to describe a casual female acquaintance.