Typically the vernacular of really degenerate
people.
Funnin means that someone has played a joke on you or made you look foolish.
Funnin can be as simple as ambushing unsuspecting
people with a paintball
gun and laughing about it or as destructive as secretly destroying a project that someone has worked very hard on at their work.
When setting someone for funnin all of the people who were witness to it are supposed to deny any knowledge of of the incident.