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Webster 1913 Edition

Adj.

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.]

ironclad

ironclad ‎(not comparable)

Covered with iron, steel, or some metal, armor-plated

1903, The Land Ironclads (Science Fiction), Digitized edition, Project Gutenberg, published 2006:
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.
(figuratively) 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.
(figuratively) Rigorous; severe; exacting.
an ironclad oath or pledge

ironclad ‎(plural ironclads)

A metal-plated ship, vessel, or vehicle

1903, The Land Ironclads (Science Fiction), Digitized edition, Project Gutenberg, published 2006:
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.
(military) An armor-plated warship.

armor-plated warship