[Source] Chen Shou (Jin Dynasty), *Records of the Three Kingdoms·Book of Shu·Biography of the First Ruler*.
[Meaning] Bi: thigh. Due to a long period without riding a horse, the flesh on the thigh has grown back. This phrase describes someone who has lived a long life of ease and comfort, wasting time and accomplishing nothing.
[Historical Anecdote]
After Liu Bei suffered defeat in battle against Cao Cao and lost his territory, he had no choice but to seek refuge with Liu Biao, a member of the Han imperial family. Liu Biao was very impressed with Liu Bei's character and conduct, so he gave Liu Bei a thousand troops and stationed him in the Xinye region (present-day Nanyang City, Henan Province). Liu Bei began once again to recuperate and build his strength in Xinye.
Cao Cao was determined to unify the north. Soon, he led his army northward to attack Yuan Shao. Liu Bei urgently advised Liu Biao to launch a surprise attack on Cao Cao's stronghold, Xudu. However, Liu Biao, seeking comfort and ease, did not accept Liu Bei's suggestion. Later, Cao Cao eliminated Yuan Shao and returned his army to Xudu. Liu Biao deeply regretted his decision. For this reason, he inwardly held Liu Bei in high esteem.
A few days later, Liu Biao invited Liu Bei to drink and chat. During the gathering, Liu Biao said to Liu Bei, "I didn't listen to your advice last time and missed a great opportunity. It's truly regrettable!" Liu Bei comforted him, saying, "Nowadays, the empire is divided, and warfare occurs daily. Just because we missed an opportunity last time, how can we know we won't encounter another in the future? Opportunities are endless. What's done is done; there's no need to dwell on past regrets." The two conversed very congenially and discussed their future plans. After a while, Liu Bei rose to use the restroom. As he touched his own thigh, he discovered that the flesh on his thigh had grown back, and he could not help but shed tears. When he returned to his seat, tear stains were still visible on his face. Liu Biao found this strange and asked, "What's wrong? Are you unwell, or do you have something troubling you?"
Liu Bei replied somewhat embarrassedly, "Nothing serious. In the past, I was constantly engaged in southern and northern campaigns, spending long periods never far from my horse's saddle, so the flesh on my thighs was lean and strong. Since coming here, five years have passed in the blink of an eye—living in leisure and comfort, with no need to ride a horse. The flesh on my thighs has regrown, now flabby and loose. Thinking of how quickly time passes, how I am growing old, and that I have yet to achieve any success in restoring the Han dynasty, my heart feels deeply distressed."
Later generations summarized this story into the idiom "regrowth of flesh on the thigh" (Bi Rou Fu Sheng), using it to express the idea of wasting time and accomplishing nothing while living for a long time in a comfortable and idle environment.