Ironclad

Clad in iron; protected or covered with iron, as a vessel for naval warfare.

Rigorous; unbreakable; severe; exacting; inflexible; as, an ironclad oath or pledge.[Colloq.]

A naval vessel having the parts above water covered and protected by iron or steel usually in large plates closely joined and made sufficiently thick and strong to resist heavy shot. Modern naval vessels are made of steel throughout, and this term is only used in historical contexts.

having an outer covering of iron or steel; as, an ironclad war vessel.

so strong or secure as to be unbreakable; as, an ironclad contract.