fortress code

Atrociously written code that is only used or kept around because other code or systems rely on it.

Person A: Work today was rough. Someone threw together a program last week that takes minutes to organize files and we were told to use it.
Person B: Are you not allowed to change it?
Person A: Nope; it's fortress code at this point. Our boss replaced the computer's file manager with the program, and any change made to it would brick the computer.