The word says its own picture out loud: iron plus clad, something wearing a coat of iron. Start with a wooden ship in the 1860s with sheets of iron bolted over its sides. Shot after shot hits it and nothing gets through. Now take the coat off the ship and lay it over a promise, a contract, an alibi: the same idea, that there is no soft spot anywhere on the outside and nothing can get in. Each meaning in this entry is that coat of iron on a different thing.
From the word’s history
From iron plus clad, where clad goes back through Middle English to Old English for clothing something. So the word is literally iron-dressed, which is why both the ship and the promise make sense under one image.
由 iron（鐵）加上 clad 組成；clad 經中古英文上溯到古英文表示「給…穿衣服」的字。所以整個字字面上就是「被鐵穿上的」。船和承諾能共用一個畫面，原因就在這裡。
Wiktionary entry for ironclad, etymology tree iron + clad, source_ref kaikki:3f6f69a190036ae4:1115046431:17758 (packet evidence)
登入就可以看每一支怎麼連回核心畫面。