four oh four

1. an unintelligent person; MORON, DIMWIT.

Example: I wouldn't ask him, he's a 404.

2. Extended to humans to convey that the subject has no idea or no clue — sapience not found. May be used reflexively:

Example: “Uh, I'm 404ing (on that)” {= “I'm drawing a blank”}.(Original source: The Jargon File.)

roughly translated — упс, я понятия не имею, туплю, не туда попал, я ошибся

from the HTTP error “file not found on server”
404 — код ошибки, когда http-сервер не может найти ту или иную страницу.