What is the meaning of Ironclad?

Covered with iron, steel, or any other tough metal; armor-plated.

Solid or certain; not able to be disputed or questioned; irrefutable.

Rigorous; severe; exacting.

Stubborn; inflexible.

A ship, vessel, or vehicle with a covering of iron, steel, or any other tough metal.

An armor-plated warship, one preceding the invention of harveyized steel.

A wooden warehouse with an outer skin of corrugated metal.

Source: wiktionary.org