Ironclad

Adjective

 * 1) Covered with iron.
 * 2)  Solid or certain; not able to be disputed or questioned; irrefutable.
 * The guy had an ironclad alibi for his whereabouts on the night of the crime.

Noun

 * 1)  A ship or vessel covered in iron.
 * 2)  An ironclad warship.

Thesaurus
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

Etymology
iron: + clad: