This is the 6511th most frequent German word.
eiserne
"Iron" or "made of iron" (figuratively: steadfast or strict).
The word 'eiserne' describes the material 'iron', used metaphorically for strength and durability.
Er trägt eine eiserne Rüstung.
He wears an iron armor.
Here, 'eiserne' metaphorically illustrates rigidity and unyieldingness of the rule.
Die eiserne Regel lautet: Nicht aufgeben.
The iron rule is: Never give up.
In this sentence, 'eisernen' figuratively characterizes her determination as strong and resilient.