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.
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)
Two branches
材料本身：鑄鐵做的
這一支就是核心畫面的字面版本：那塊倒模冷卻出來的鐵，被拿去做成鍋子、水管、欄杆。
adjective · B2
Made of cast iron, a hard heavy metal that is shaped by pouring it into a mould while it is melted. Things made this way, such as pans, pipes and gates, are thick and last a long time.
鑄鐵製的
比喻：弄不破的保證
同一塊鐵，換個地方放。不是拿來做鍋子，而是拿來形容一句承諾、一個理由有多硬。
adjective · C1
Used in front of words like guarantee, excuse, alibi, case and rule to say that the thing is completely certain: nothing can break it, disprove it, or make it fail.
鐵打的；絕對可靠的