eisern

Etymology
From, , from. Equivalent to, older variant of. Now reanalysable as, but historically is itself one of the words that contributed to the development of this extended suffix  (see there).

Adjective

 * 1) iron, ironclad, made of iron
 * 2)  staunch, unyielding

Usage notes

 * As usual, Modern German prefers compound nouns to the adjective construction, e.g. rather than . Therefore the figurative use is now predominant.