To mistake the reflection of a bow in one's cup for a snake
To scare oneself by being overly suspicious or paranoid; to be jumpy
Negative
自从上次从世纪欢乐园的"鬼屋"中出来，他就一直杯弓蛇影 | Zìcóng shàng cì cóng shìjìhuānlèyuán de "guǐwū" zhōng chūlái, tā jiù yīzhí bēigōngshéyǐng
Ever since he came out of the haunted house at Century Fun Park, he's been extremely jumpy
[Source]
不过，历经30年压制，新闻记者早已杯弓蛇影 | Bùguò, lìjīng 30 nián yāzhì, xīnwén jìzhě zǎoyǐ bēigōngshéyǐng
However, after thirty years of oppression, news reporters have long been extremely suspicious and nervous
[Source]
None
None
乐 is a surname in this story and is pronounced as "yuè," not "lè."
《晋书》(Jìn Shū), a work documenting the history of the Jìn Dynasty (晋, 265 - 420). It was written by Táng Dynasty (唐, 618 - 907) historian 房玄龄 (Fáng Xuán Líng)
乐广 (Yuè Guǎng) was a government official from Hénán (河南). One day, he asked a dear friend why he hadn't been over to see him in a few days.
The friend replied, "When I was over last time, you were kind enough to give me some wine. However, when I lifted my cup, I saw that there was a snake inside of it. I grew very worried, and after I finished drinking, I fell extremely ill."
乐广 was baffled as to how a snake had gotten into his friend's cup, and after his friend left, he started looking for possible explanations. Soon, his gaze fell upon a bow that was hanging on the wall. The bow had a snake painted on it, and he realized that the image of the snake could be reflected into the drinks of those who stood near it. With that mystery solved, he invited his friend back for more drinks.
When his friend arrived, they sat in the same place as when his friend had seen the snake. 乐广 asked him whether he was still seeing a serpent in his cup, to which he responded that he was, so 乐广 promptly showed him the bow on the wall. The friend immediately realized what had transpired, and he started to feel healthy once more.