一yī落luò千qiān丈zhàng

Literal: in one fall, a thousand zhang
Figurative: To plummet all at once: reputation, standing, grades, or sales suddenly collapse from a great height, rather than slowly sliding.

Where does 一落千丈 come from?

It comes from a poem by the Tang writer Han Yu, 《听颖师弹琴》, describing the monk Ying playing the qin. To convey the music he uses images: the melody climbs until it cannot rise another inch and then, losing its footing, drops more than a thousand zhang in one go (a zhang is about three meters). It was the sudden leap of the sound from the highest notes to the lowest. The image later passed into life and business: today it is said of reputations, positions, grades, or sales that collapse all at once, not bit by bit as with 每况愈下.

Source (出处): 韩愈《听颖师弹琴》

How is 一落千丈 used? Two examples

他tā以yǐ前qián是shì班bān里lǐ的de尖jiān子zi，高gāo三sān那nà年nián家jiā里lǐ出chū事shì，成chéng绩jì一yī落luò千qiān丈zhàng。
He used to be one of the top students in his class, but in his final year something happened at home and his grades plummeted.
质zhì量liàng问wèn题tí一yī曝bào光guāng，这zhè个ge牌pái子zi的de销xiāo量liàng一yī落luò千qiān丈zhàng。
The moment the quality problem came to light, the brand's sales fell off a cliff.

In Wanyu, all 400 chengyu have audio on every sentence, and you can save them to your decks and review them with spaced repetition. Free.