1. (a.) Clad in iron; protected or covered with iron, as a vessel for naval warfare.

2. (a.) Rigorous; severe; exacting; as, an ironclad oath or pledge.

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.

armed cap-a-pie armor-plated armored constant dour firm fixed fundamentalist hard hidebound immovable immutable impliable in armor in complete steel in harness inalterable inexorable inflexible invariable iron ironbound ironhanded loricate loricated mailclad mailed muscle-bound obdurate obstinate orthodox panoplied procrustean purist puristic puritan puritanic relentless rigid rigorist rigoristic rigorous rockbound stiff straightlaced straitlaced stubborn unalterable unbending uncompromising unmodifiable unrelenting unyielding