An undocumented BigBallOfMud written for the job security of the current programmer, who can allegedly fix it, usually with more DuctTape.
I also heard it referred to as the job shield or diplomatic immunity. The trick is to create a utility or a middleware project in your spare time which is removed from company policies and documentation regulations (guaranteeing the "no one except you knows how it works" part). The difficult bit is smuggling it in and getting it embedded into your company's systems. Choosing the right moment to present it to the lead is crucial but since you work there you know exactly when the project is in need of cutting some corners and getting some extra progress. What you are doing is essentially acting as a middleware vendor but selling it for free except you get a lifetime contract for supporting it. Bad business if you really were a vendor but you're not really selling anything here - you are buying some job security. It is also the WrongThing to do.
See also: PrisonForCustomer?.