Source : Webster's Revised Unabridged Dictionary (1913)

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

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

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.

Source : WordNet®

ironclad
     adj 1: inflexibly entrenched and unchangeable; "brassbound
            traditions"; "brassbound party loyalists"; "an
            ironclad rule" [syn: {brassbound}]
     2: without flaws or loopholes; "an ironclad contract"; "a
        watertight alibi"; "a bulletproof argument" [syn: {unassailable},
         {unshakable}, {watertight}, {bulletproof}]