It comes from Ying Shao's Custom Tong in Han Dynasty, and it can also be found in Jin Shu and Yue Guang Chuan.
Le Guang once had a very good friend, who hadn't come for a long time. Yue Guang asked him why he had come. The guest replied, "Last time I played here, you bought me a drink. I was about to drink when I suddenly saw a snake in the cup, and I was disgusted at that time. I got sick after drinking it. "
At that time, a bow was hung on the wall of Le Guang's house, and a snake was painted on it. Yue Guang guessed that the snake in the cup that the guests saw might be the shadow of this bow. So he poured another glass of wine, put it in its original position and asked the guest, "Do you see anything in this wine?"
The guest replied, "I saw the same thing as last time." Yue Guang explained the reason to the guest, making him understand that the snake in the cup is just the reflection of the bow. The mystery in the guest's mind was solved at once, and the long-standing problem was soon cured.
This allusion means that the guest sees the bow shadow in the cup and thinks it is a snake in the wine, so he just drinks it. That is, I got sick because of my own doubts. When I learned the truth, my doubts disappeared and I recovered. Later, I used a cup bow snake shadow or a bow snake shadow to refer to my doubts and fears caused by hallucinations, as a metaphor for being suspicious and worried about myself. A Qing poet Huang Zunxian's poem "Feeling Things" said: "The golden bow snake shadow contains hidden pain, and the cup bow snake shadow bears strange resentment." Zhao Yi's poem "Seventy Readings": "Seeing crabs in the water is still full of breath, and the bottom of the cup is suitable for snakes."