废寝忘食 forgetting to sleep and eat
Explanation
形容专心致志，刻苦努力到忘记了吃饭睡觉的程度。
It describes someone who is so engrossed in something that they forget to eat and sleep.
Origin Story
春秋时期，孔子周游列国，来到楚国叶邑。叶县大夫沈诸梁热情接待孔子，却对孔子的学说不太了解，便私下问子路。子路一时答不上来，就去问孔子。孔子说："我为了研究学问，废寝忘食，甚至忘记了时间，一心扑在学问上。",
During the Spring and Autumn period, Confucius traveled to various states and arrived in Yeyi, Chu. Shen Zhuriang, the official of Ye County, warmly received Confucius, but he didn't quite understand Confucius' teachings, so he secretly asked Zilu. Zilu couldn't answer immediately, so he went to ask Confucius. Confucius said: "I am so engrossed in my studies that I forget to eat and sleep."
Usage
多用于形容人学习或工作十分努力，废寝忘食地投入其中。
It is mostly used to describe how immersed someone is in their studies or work.
Examples
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为了完成这项任务，他废寝忘食，夜以继日地工作。
wèile wánchéng zhè xiàng rènwu, tā fèiqǐn wàngshí, yèyǐjì rì de gōngzuò
In order to complete this task, he worked day and night, forgetting to eat and sleep.
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他最近迷上了研究古代历史，废寝忘食地阅读各种史料。
tā zuìjìn mí shàngle yánjiū gǔdài lìshǐ, fèiqǐn wàngshí de yuèdú gè zhǒng shǐliào
He recently became fascinated with the study of ancient history and read various historical materials day and night, forgetting to eat and sleep.