When combined, these characters create the graphic and metaphorical image of “inserting knives into both ribs.” This self-inflicted, painful act symbolizes the ultimate sacrifice one is willing to make out of loyalty and friendship.
The idiom `两肋插刀` is deeply rooted in the Chinese concept of `义气 (yìqi)`, a complex term that blends righteousness, honor, loyalty, and a code of conduct, especially among close friends or “sworn brothers” (好兄弟). This value is a cornerstone of classic Chinese literature like Water Margin (《水浒传》) and countless wuxia (武侠) novels and films, where heroes are defined by their unwavering loyalty to their comrades.
This is a highly expressive and somewhat dramatic idiom. It's not typically used for small favors.