Once, there was a man named Sun Yan who had been married for three years. His wife was beautiful and virtuous, but one thing always puzzled him: she never took off her clothes when sleeping. Several times, Sun Yan wanted to ask why, but his wife always "looked left and right and talked about other things," changing the subject.
Sun Yan was determined to solve the mystery. One day, he pretended to leave on business, telling his wife to sleep first. After a while, he quietly returned home, entered the room, and saw his wife fast asleep. Tiptoeing to the bedside, he secretly unfastened her clothes. What he saw terrified him out of his wits—behind his wife trailed a fox's tail over three feet long!
Legend has it that a well-practiced fox can transform into human form, but its tail remains stubbornly unchanged, often giving the creature away.
The idiom "fox's tail" is used to describe the evidence or traces left behind by a villain's true nature or deceitful schemes.
Source: Yang Xuanzhi (Northern Wei Dynasty), *A Record of Buddhist Monasteries in Luoyang*
Meaning of the Idiom: Later, the Chinese idiom "狐狸尾巴" came to describe evidence or traces left behind by a villain's true nature or deceitful schemes.