推心置腹

Origin and story

After Liu Xiu defeats the Tongma forces, the Eastern Han records describe him accepting their surrender and granting positions to their leaders. Distrust remains on both sides. He tells the surrendered men to return to their camps and then enters to inspect them with a light mounted escort, demonstrating that he does not expect treachery.

The men respond that the Prince of Xiao has placed his sincere heart inside others' breasts, asking how they could fail to give themselves in return. The later idiom compresses that vivid declaration. It means treating someone with wholehearted sincerity and trust, not literally sharing an organ or merely exchanging confidential gossip.