An outside joke is actually an inside joke, the difference being this: an insider is shared between a few close friends, and an outside joke (also referred to as outsider) is shared among a public amount of people, based on a video posted on YouTube, a celebrity's fanpage on Facebook, a funny line from Family Guy, etc.
Jackie: Did you see that movie Forgetting Sarah Marshall?
Mark: Yes! Remember that part where...
Jess: Yeah when Peter's all upset and...
Andre: *laughs* I loved that movie.
GROUP: *starts singing Dracula's Lament*
Jackie: Guys, this movie is totally becoming our outside joke.
From amusing to terrifying, outside jokes incisively reveal a speaker's deficiency and usually highlight their gifuckingnormous douchebaggery. While different eras would have forgiven or simply forgotten such excretory gems (you can't make this shit up), the rise of internet blogging, cutesy Daily Show quips and a President who planned 9/11 in alliance with Israel (see this is an outside joke!), The oughts became larded with pathetic irony purveyors. Though we may have left the golden age (or festering shit swamp) of outside jokes, examples are available anytime Barack Obama expresses outrage, a college student describes themselves as spiritual but not religious, or when a middle aged basketball scout salivates over a nineteen year old "physical specimen".
I miss Tom Delay for his outside jokes.