ironclad

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Sheathed with iron plates for protection.
2. adj. Rigid; fixed: an ironclad rule.
3. n. A 19th-century warship having sides armored with metal plates.

Century Dictionary and Cyclopedia

1. Covered or cased with iron plates, as a vessel for naval warfare; armor-plated.
2. 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.
3. n. 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. The metal armor is often of great thickness; over parts of H. M. S. Inflexible, for example, the metal is as much as 24 inches thick. Even the thickest armor used, however, is not sufficient to keep out the projectiles of the high-pressure guns of the present day; moreover, its great weight prevents the application of heavy armor except to the most vulnerable parts of the ship. The flrst armored vessels were built by the French for use during the Crimean war, and the success of the monitors during the civil war in the United States gave a strong impetus to the building of ironclads. Iron-clad ships are now made of very various designs. Many modern vessels have protective iron decks, but the term iron-clad has been confined to vessels whose sides are protected. Iron-clad ships are generally armed with two or four heavy breech-loading rifled guns of from 10 to 16 inches caliber, in addition to a secondary battery of smaller breech-loading and rapid-flring guns. They are usually constructed as rams, and their hulls are divided into numerous water-tight compartments.
4. 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.

Wiktionary

1. adj. Covered with iron, steel, or some metal, armor-plated
2. adj. figuratively Solid or certain; not able to be disputed or questioned; irrefutable.
3. n. A metal-plated ship, vessel, or vehicle
4. n. military An armor-plated warship.

GNU Webster's 1913

1. adj. Clad in iron; protected or covered with iron, as a vessel for naval warfare.
2. adj. colloq. Rigorous; unbreakable; severe; exacting; inflexible.
3. n. 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.
4. adj. having an outer covering of iron or steel.
5. adj. so strong or secure as to be unbreakable.

WordNet 3.0

1. n. a wooden warship of the 19th century that is plated with iron or steel armor
2. adj. sheathed in iron plates for protection
3. adj. inflexibly entrenched and unchangeable