死心塌地 devoted heart and soul
Explanation
原指死了心，没有别的想法；后指下定决心，再不改变。形容对某人或某事非常忠诚，坚定不移。
Originally meant to have given up hope, no other ideas. Later on it refers to making a firm decision and not changing it. It is used to describe a person who is very loyal to someone or something and unwavering.
Origin Story
话说河南府尹李彦实与爱女玉英相依为命，却遭人诬陷，急需十两银子去受审。他向当地富豪刘员外借钱，却被刘员外趁机逼迫玉英嫁给他。玉英早已与进京赶考的书生张晋卿私定终身，两人情投意合，两情相悦。张晋卿高中状元后回乡，任职洛阳县令，他得知玉英的遭遇后，立刻赶到刘员外府上，与玉英相认。为了救回心爱的玉英，他承诺不惜任何代价，一定赎回玉英。他先设法还清了玉英欠刘员外的债务，最终，有情人终成眷属。张晋卿对玉英一片真心，对他们的爱情死心塌地，最终，他们也过上了幸福快乐的日子。
It is said that Li Yanshi, the prefect of Henan Prefecture, lived with his beloved daughter Yuying, but was wrongly accused and urgently needed ten taels of silver for his trial. He borrowed money from the wealthy Liu landlord, but Liu took the opportunity to force Yuying to marry him. Yuying had secretly become engaged to the scholar Zhang Jinqing, who went to the capital for the examination. The two were in agreement and loved each other. After Zhang Jinqing became the highest-ranking official, he returned home and took office as the magistrate of Luoyang. After learning about Yuying's plight, he immediately rushed to Liu's mansion to meet with Yuying. To rescue his beloved Yuying, he promised to pay any price to redeem her. He first tried to repay Yuying's debts to the Liu landlord. In the end, the lovers were together. Zhang Jinqing was sincere to Yuying and devoted to their love; in the end, they lived happily ever after.
Usage
表示对人或事极其忠诚、坚定不移；多用于褒义。
It is used to express extreme loyalty and unwavering devotion to a person or thing; mostly used in a positive way.
Examples
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他对她死心塌地，从不怀疑她的忠诚。
ta dui ta si xin ta di, cong bu huai yi ta de zhong cheng.
He was devoted to her and never doubted her loyalty.
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为了理想，他死心塌地地奋斗了十年。
wei le li xiang, ta si xin ta di de fen dou le shi nian.
For his ideal, he had worked hard for ten years with all his heart and soul.