dead leg - A hot-water pipe leading to a tap and not part of a circuit. The hot water in a dead leg cools off between draw-offs, wasting energy and heat, and increasing the danger of legionnaire's disease. Domestic hot water cylinders are located near to the use points so as to limit dead legs to the lengths required by water companies. Large installations often have secondary circuits, but even they have short dead legs.
[Scott/Penguin Dict. of Building]