ironclad

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Sheathed with iron plates for protection.
• adjective Rigid; fixed.
• noun A 19th-century warship having sides armored with metal plates.

from The Century Dictionary.

• Covered or cased with iron plates, as a vessel for naval warfare; armor-plated.
• Figuratively, very rigid or strict; constructed, as a form of words, so as to allow no evasion or escape, or permit no flaw to be detected.
• Noting an electrical apparatus or machine in which the iron part of the structure completely or partly surrounds and thereby mechanically protects the electric conductors: as, an iron-clad armature, one having the conductors embedded in slots or holes.
• noun A naval vessel cased or covered wholly or partly with thick iron or steel plates, generally having a heavy backing of wood, so armored to resist projectiles or the attacks of rams or other armored vessels.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• adjective having an outer covering of iron or steel.
• adjective so strong or secure as to be unbreakable.
• adjective Clad in iron; protected or covered with iron, as a vessel for naval warfare.
• adjective colloq. Rigorous; unbreakable; severe; exacting; inflexible.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Covered with iron, steel, or some metal, armor-plated
• adjective figuratively Solid or certain; not able to be disputed or questioned; irrefutable.
• noun A metal-plated ship, vessel, or vehicle
• noun military An armor-plated warship.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a wooden warship of the 19th century that is plated with iron or steel armor
• adjective sheathed in iron plates for protection
• adjective inflexibly entrenched and unchangeable