说来话长 It's a long story
Explanation
表示事情很复杂，不是几句话就能说清楚的，多指不大愉快的事。
indicates that the matter is very complex and cannot be explained in a few words, often referring to unpleasant matters.
Origin Story
老张是一位经验丰富的工程师，一天，他被叫到项目经理办公室。“老张，这个项目出了问题，你来看看吧。”项目经理愁眉苦脸地说。老张仔细查看了项目文件，发现问题确实棘手，牵涉到多个部门、多个环节，还有复杂的国际合作。他沉默了一会儿，然后对项目经理说："说来话长，这个情况很复杂，需要我仔细研究，明天才能给您答复。"
Old Zhang was an experienced engineer. One day, he was called to the project manager's office. "Old Zhang, there's a problem with this project, come and have a look." The project manager said with a worried look. Old Zhang carefully examined the project documents and found that the problem was indeed thorny, involving multiple departments, multiple links, and complex international cooperation. He was silent for a while, and then said to the project manager: "It's a long story, this situation is very complicated, I need to study it carefully, and I can give you an answer tomorrow."
Usage
作谓语、分句；用于口语，指不大愉快的事。
Used as a predicate or clause; used in colloquial language, often referring to unpleasant matters.
Examples
-
事情很复杂，说来话长。
shì qing hěn fù zá, shuō lái huà cháng
The matter is quite complicated; it's a long story.
-
这其中的原因说来话长，一言难尽。
zhè qí zhōng de yuányīn shuō lái huà cháng, yī yán nán jìn
The reason is complicated; it's a long story.