It's when you make jokes with your self. Usually in one's head, but sometimes out loud and then no one understands. Also shortened to humorbate for the action, and humorbator for the reference of one in the action.
While waiting in line, I was humorbating to keep myself in a good mood and pass time.
When I turned into the aisle, the employee was laughing with noone around, he's a humorbator.
Sometimes I'm so bored I humorbate.