When someone, or a group, acts as though they are doing something amazing, or phenomenal, or "really meaningful" or important, when to everyone else they are just humiliating themselves.
Based on the characters in the movie, "Waiting for Guffman," or any of Christopher Guest's ensemble films.
My biggest Guffman Moment was in high school when I auditioned to be one of the male cheerleaders, while wearing a self-designed costume with puffy sleeves and gold lame' short shorts. At my 20th reunion, they were still teasing me about it.
At some point George Bush is going to realize that his whole life was a Guffman Moment.