<abuse> Someone who's clueless. From the World-Wide Web message "404, URL Not Found" meaning that the document you've tried to access can't be located.
"Don't bother asking him...he's 404, man".
404 is one of the standard response codes of the telnet protocol on which the web's HTTP is based.
The first 4 indicates a client error such as a mistyped URL. The middle 0 refers to a general syntax error. The last 4 just indicates the specific error in the group of 40x, which also includes 400: Bad Request, 401: Unauthorized, etc.
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Nearby terms: 3NF « 3Station « 3-tier « 404 » 4.2BSD » 431A » 4.3BSD