During the late Eastern Han Dynasty, a Xiongnu envoy arrived to meet the King of Wei—Cao Cao. Believing his own appearance too plain to command respect from foreign dignitaries, Cao Cao ordered his military officer Cui Yan to act as his stand-in. Cui Yan, with his handsome features, clear brow, and flowing long beard, cut an impressively majestic figure.
During the audience, Cui Yan wore Cao Cao's robes and cap, looking even more spirited than usual. Cao Cao himself, however, stood respectfully beside Cui Yan's throne, holding a sword and pretending to be a guard.
After the meeting, Cao Cao sent someone to learn the Xiongnu envoy's reaction to the King of Wei. The envoy said, "The King of Wei has an exceptionally refined appearance. However, the sword-bearer by his bedside is the true hero."
Based on this story, later generations called ghostwriting "holding the knife," and the person who writes for another is called a "knife holder." This also led to the idiom "holding the knife as a proxy," meaning to write on someone else's behalf.
Source: *A New Account of the Tales of the World*, "Appearance and Behavior"
Meaning of the Idiom: Later, the Chinese idiom "捉刀代笔" came to describe how to write on someone else's behalf.