The act of desecrating a wikipedia page with false information, thereby fucking over anybody who decides to use information from that page within the near future. In rare instances, Wikigraffiti can go unnoticed for weeks or months at a time. This act can be quite fun, and there are games based on it as well.
Noun - When in an instance when you need to prove someone wrong, just pull some Wikigraffiti and you'll be safe.
Verb - It is pretty typical of people to Wikigraffiti the "George Washington" page on Wikipedia.
Wayne - "Hey, for homework our History Professor told us to get on wikipedia and mess with all of the definitions just to show how unreliable it is!"