When a nerd tries to hit on someone out of his league and gets rejected. The nerd is therefore unable to woo the person he is hitting on. Frequently comes in the form of a disgusted facial expression or the person walking away from the nerd looking like they saw a ghost.
Named after the infamous 404 web page error that occurs when the page is unavailable.
Alex tried to offer the woman a beer but instead got a 404 - Unable to Woo when he approached her.