Best definition
The excuse a person would invent to leave a social gathering or event to go play World of Warcraft

Your Friend Tommy is at a party, 10 minutes into the party he says he has to go home and clean his room. Later you find out that he left the party to go play World of Warcraft, and he just needed an excuse to leave and play the game.

Cleaning his room was his wowlibi, as it was the excuse that allowed tommy to leave and play world of warcraft