ironclad

Etymology

From iron + clad.

adj

1. Covered with iron, steel, or some metal; armor-plated.
   Unlike the average vehicle, cash delivery vans are ironclad and almost unstoppable.
   In that flickering pallor it had the effect of a large and clumsy black insect, an insect the size of an ironclad cruiser, crawling obliquely to the first line of trenches and firing shots out of portholes in its side. 1903, The Land Ironclads, Digitized edition (Science Fiction), Project Gutenberg, published 2006
2. (figurative) Solid or certain; not able to be disputed or questioned; irrefutable.
   The suspect had an ironclad alibi for his whereabouts on the night of the crime.
3. (figurative) Rigorous; severe; exacting.
   an ironclad oath or pledge
4. (figurative) Stubborn; inflexible.

noun

1. A metal-plated ship, vessel, or vehicle.
   He turned again to the nearest land ironclad, advancing now obliquely to him and not three hundred yards away, and then scrambled the ground over which he must retreat if he was not to be captured. 1903, The Land Ironclads, Digitized edition (Science Fiction), Project Gutenberg, published 2006
2. (military) An armor-plated warship, (especially) one preceding the invention of harveyized steel.
   About a couple of miles out lay an ironclad very low in the water, almost, to my brother's perception, like a water-logged ship. This was the ram Thunder Child. 1898, H.G. Wells, The War of the Worlds, London: William Heinemann, page 178