离题万里 Miles off the topic
Explanation
形容说话或写文章偏离主题很远，与主题毫无关系。
Describes speech or writing that deviates far from the topic and has no relation to the subject.
Origin Story
从前，有个秀才去参加科举考试。主考官出的题目是：‘赋得古原草送别’。秀才胸有成竹，提笔就写，可是写着写着，他竟写起了自己家乡的风土人情，然后又写到自己儿时的趣事，最后竟然写到了他昨天晚上做的一个梦。考官读罢，哭笑不得，批道：‘下笔千言，离题万里’。
Once, a scholar went to take the imperial examination. The examiner's topic was: 'A poem about the grass of the ancient plain, saying goodbye'. The scholar was confident and began to write, but as he wrote, he wrote about the customs and folkways of his hometown, then about his childhood anecdotes, and finally even about a dream he had last night. The examiner laughed and cried at the same time after reading it, and wrote: 'A thousand words written, but miles away from the topic'.
Usage
常用来批评说话或作文离题太远，与主题不相关。
Often used to criticize speech or writing that deviates too far from the topic and is unrelated to the subject.
Examples
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他这番话，完全是离题万里，与主题毫无关系。
ta zhe fan hua, wanquan shi li ti wan li, yu zhut ti hao wu guanxi.
His words were completely off-topic and irrelevant to the subject.
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这篇论文离题万里，论证不充分。
zhe pian lunwen li ti wan li, lunzheng bu chongfen
This paper is miles off-topic and lacks sufficient evidence.