The Collaborative International Dictionary of English v.0.48:

Ironclad \I"ron*clad`\, a.
   1. Clad in iron; protected or covered with iron, as a vessel
      for naval warfare.
      [1913 Webster]

   2. Rigorous; unbreakable; severe; exacting; inflexible; as,
      an ironclad oath or pledge. [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ironclad \I"ron*clad`\, 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.
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Ironclad \I"ron*clad`\, a.
   1. having an outer covering of iron or steel; as, an ironclad
      war vessel.
      [PJC]

   2. so strong or secure as to be unbreakable; as, an ironclad
      contract.
      [PJC] irone

WordNet (r) 3.0 (2006):

ironclad
    adj 1: sheathed in iron plates for protection
    2: inflexibly entrenched and unchangeable; "brassbound
       traditions"; "brassbound party loyalists"; "an ironclad rule"
       [syn: ironclad, brassbound]
    n 1: a wooden warship of the 19th century that is plated with
         iron or steel armor

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "ironclad":
   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