During the late Eastern Han Dynasty, Liu Bei was defeated by Cao Cao in Runan, left with only a few hundred soldiers and nowhere to turn. His general Sun Qian advised him to temporarily seek refuge with Liu Biao, the governor of Jingzhou. Liu Bei went to Jingzhou, and Liu Biao took him in.
One day, Liu Bei and Liu Biao were drinking together, and as the wine warmed their spirits, Liu Bei suddenly felt unwell and excused himself to the privy. While loosening his clothes, he noticed the flesh on his inner thighs had grown plump again, and tears streamed down his face.
Liu Bei returned to the living room. Liu Biao, seeing his dejected expression, asked, "What unpleasant thing have you encountered?" Liu Bei shook his head and did not reply.
Liu Biao was puzzled. Just moments ago, the man had been chatting and laughing freely, so why was he now frowning with a furrowed brow?
"What's really on your mind?"
Liu Bei let out a long sigh and said, "In the past, I fought tirelessly to defeat the rebels and restore the Han dynasty, always in the saddle, until the flesh on my thighs wasted away. Now, I live here idly, eating my fill each day with nothing to do, and the flesh on my thighs has grown back. I am growing old without having achieved any great deeds—how can this not grieve me!"
Later, the idiom "Sigh of the Thigh Flesh" came to describe someone who, long idle, laments unfulfilled ambitions and yearns to achieve something.
Source: *Records of the Three Kingdoms*, "Biography of the First Lord of Shu"
Meaning of the Idiom: Later, the Chinese idiom "脾肉之叹" came to describe how someone who long idle laments unfulfilled ambitions and yearns to achieve something.