The area between a normal human being's large intestine and colon.
An extremely important portion of the digestive system that schools around the globe refuse to reveal the existence of to students. Usually, when a semi-colon is in any way damaged or deformed, it can become a major threat to the owner's life (if left untreated, you could shit out your intestines).
Fred: Dude, my doctor said I have an enlarged semi-colon.
Chris: Do you know what that means?
Fred: No, what?
Chris: It means someone role-played PowerMan and IronFist in your ass.
Fred: Oh shit.
Chris: Yeah. You're going to need a semi-colonoscopy.
Fred: I hate asking all these questions, but WHAT is THAT?
Chris: Your doctor is going to get a rabbi to bless your asshole and then stick a very hot shaft of PVC pipe so far up your butt it'll pop out your eyeballs. Then, they'll procede to stuff as many red permanent markers up into the pipe, then flowing into your skull, to make sure the pipe went all the way through, and tell you that you have been exposed to a treatment that still has not passed through the appropiate legal channels, and that you are not liable to sue. If you did somehow threaten to sue, they will threaten you by saying there is no way to remove the pipe from your body unless they do it themselves, and if you do sue, you will have to walk around with a pipe in your ass for the rest of your life.
Fred: Oh shitter.