What is Cast iron ?
Cast iron is (noun) 1. iron which is shaped in a mould, not bent, and so breaks easily The pipes are made from cast iron, and are very solid. Don’t let the cast-iron pan soak in water as it will rust. Compare wrought iron 2. cast-iron excuse a very good excuse, one that cannot be turned down He has a cast-iron excuse and won’t get into trouble with the authorities.
source: Easier English, Student Dictionary Upper Intermediate Level