Picture melted iron being poured into a mould and left to cool. What comes out is one solid, heavy piece: thick, cold and stubborn. It does not bend when you lean on it, and an old cast-iron gate is still standing long after the wooden one beside it has gone. Both uses of the word live on that one object. In front of pan, pipe or it names the metal itself; in front of guarantee, excuse or rule it lends that metal's refusal to bend to a promise or a reason.
可以想成一鍋燒紅的鐵水，倒進模子裡，慢慢冷掉。拿出來是一整塊實心的東西：又厚、又重、又硬。你壓它，它不彎；放在外面幾十年，它還在。cast-iron 的兩個用法都貼著這個畫面。放在 pan、pipe、railing 前面，講的是這個材料本身；放在 guarantee、excuse、rule 前面，借的是它「弄不壞、折不彎」的那股勁。
From the word’s history
The word is its own picture: iron that has been cast, that is, poured into a mould while molten.
這個詞本身就是畫面：cast 是澆鑄，iron 是鐵，合起來就是「倒進模子裡成形的鐵」。
transparent compound of cast + iron; supported by the packet's own alt-form relation cast-iron ↔ cast iron (Wiktionary)