Leave it to South Park to choose a condition so rare that it’s only been documented 87 times.
Fetus in fetu is just like the name sounds–a fetus inside a fetus. At some point during development, so the hypothesis goes, the cells that would normally form an identical twin somehow start to develop inside the other embryo. The “parasitic twin” actually develops some basic organs in some cases, as well as a vertebral column.
Update, 1/28/03: I’ve just gotten about 50 hits for “fetus in fetu.” Someone wanna comment and tell me why it’s such a big term in the last 2 hours?