Legacy System
Old software that still runs something critical. 'Legacy' is pejorative until you try replacing it, at which point it's revealed as load-bearing and undocumented — the building's mystery wall.
Heard in the wild
“Payroll runs on a legacy system three retirements away from unmaintainable.”
Tone check: Danger
Every 'quick legacy replacement' estimate is wrong by 3x. The system knows things that were never written down.