Green’s Dictionary of Slang

(US teen) a fool.

1997–2000		Da Bomb 🌐 1: 404: Someone who is clueless or naive (derived from www message ‘404, URL not found’). Don’t ask him about the homework because he is.
2000		N.Y. Times 27 June n.p.: Calling someone a ‘404’ (from the World Wide Web error message, ‘404 Not Found’) means he is clueless or has a high ‘bozon count,’ while accusing him of being a ‘BDU’ means he’s a ‘Big Dumb User.’.