Wang Xizhi, the great calligrapher of the Eastern Jin Dynasty, was the nephew of Chancellor Wang Dao and displayed remarkable poise even in his youth. Wang Dao's other nephews and sons were also highly talented.
At the time, Grand Tutor Xi Jian had a daughter and wanted to find her a worthy husband. Hearing that the Wang family's sons were all talented and handsome, he sent a student with a letter to Wang Dao, requesting to select a son-in-law from among the Wang clan. Wang Dao readily agreed and invited Xi Jian's envoy to come and choose.
The student relayed Wang Dao's opinion to Xi Jian, who said to the student, "Go again. Take a good look at the Wang family's sons and report back to me what you see."
Then, this student went to the Wang family to help Xi Jian select a son-in-law. Wang Dao ordered all the young men to the eastern wing and said to Xi Jian's student:
"Go ahead and pick whichever you like!"
When the young men of the Wang family heard that Grand Tutor Xi was coming to select a son-in-law, they all dressed in fine clothes, groomed themselves meticulously, and sat upright in the eastern wing, looking solemn and proper. Only Wang Xizhi lay on his bed, with his robe lifted up, baring his belly, holding a flatbread in his hand, eating it bite by bite, as if nothing were happening.
After returning, the student reported to Xi Jian:
"The Wang family's young men are all quite fine. When they heard the Grand Tutor was coming to choose a son-in-law, they all acted very proper. Only one lay on the bed with his belly exposed, as if he hadn't heard." Xi Jian said happily, "This man is exactly the one to be my son-in-law."
So Chi Jian chose Wang Xizhi and married his daughter to him.
Later, the idiom "East Bed Bare Belly" came to refer to a son-in-law or the act of selecting one.
Source: *A New Account of the Tales of the World*, Chapter "Magnanimity"
Meaning of the Idiom: Later, the Chinese idiom "东床坦腹" came to describe a son-in-law or the act of selecting one.