404 is used to describe someone who is clueless. This usage derives from the WWW error message, “404 Not Found”, meaning the requested document could not be located.

404被用来描述愚笨、无用之人。这一用法来源于网络上无法找到搜索页面时出现的“404 Not Found”错误提示信息。

Someone 404 is usually stupid and unable to use computers or other complicated electronic equipment.

被称为404的人通常比较笨而且对电脑或其他复杂电器的操作也不太开窍。

For example:

Don't bother asking him, he's 404.

不用去问他了，他什么都不会。