I haven't a clue
Originally a technical term for Not Found 404 (which is an error message seen on a Web page to indicate a requested URL was not found on a server), in slang to say "404" is to imply someone is clueless, as in "There's no use asking him; he's 404, dude."
Online jargon, also known as text message shorthand, used primarily in texting, online chat, instant messaging, email, blogs, and newsgroup postings, these types of abbreviations are also referred to as chat acronyms.
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NetLingo Classification: Acronyms and Text Message