Having a crazy-ass time.
Why you trippin' bout dat neck-slap, son? I was just funnin'!
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.
We was just funnin with him when we took the front wheel off of his bicycle and threw it in the river.