
eisern
ironclad
In a figurative sense: uncompromising, strict, strong-willed, or unwavering.
Sie verfolgte ihr Ziel mit eiserner Disziplin.
She pursued her goal with ironclad discipline.
iron / ironclad
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ironclad
In a figurative sense: uncompromising, strict, strong-willed, or unwavering.
Sie verfolgte ihr Ziel mit eiserner Disziplin.
She pursued her goal with ironclad discipline.

iron
Made of the metal iron.
Das schwere, eiserne Tor ließ sich nur langsam öffnen.
The heavy iron gate could only be opened slowly.

emergency reserve
A reserve of money or goods that is only used in the most extreme emergency.
Für unvorhergesehene Reparaturen am Haus haben wir eine eiserne Reserve.
We have an emergency reserve for unexpected repairs to the house.
'Eisern' (iron / ironclad) is the adjective derived from the noun 'das Eisen' (iron). The figurative meaning comes from the properties of the metal: hard, strong, and unbending.
Er hat einen eisernen Willen und gibt niemals auf.
He has an iron will and never gives up.
You can recognize the meaning from the noun. If 'eisern' (ironclad) refers to an abstract concept like 'Wille' (will), 'Disziplin' (discipline), or 'Regel' (rule), the figurative meaning is intended. If it refers to an object like 'Tor' (gate) or 'Kette' (chain), it refers to the material.
Er befolgt die Regeln mit eiserner Konsequenz.
He follows the rules with ironclad consistency.
In its figurative meaning, 'eisern' (ironclad) is often combined with abstract nouns to express strength or uncompromising firmness. Common combinations are: 'eiserner Wille' (iron will), 'eiserne Disziplin' (ironclad discipline), and 'eisernes Schweigen' (stony silence).
Trotz der Provokationen bewahrte er ein eisernes Schweigen.
Despite the provocations, he maintained a stony silence.
The expression 'eiserne Reserve' (emergency reserve) is a fixed phrase for emergency savings or supplies that may only be used in an emergency. Here, 'eisern' means 'untouchable'.
Jeden Monat lege ich 50 Euro für meine eiserne Reserve zur Seite.
Every month I set aside 50 euros for my emergency reserve.
hard
When describing strict or uncompromising measures, both „hart“ and „eisern“ can be used. „Hart“ emphasizes the severity and possible burden of a measure, while „eisern“ emphasizes the unshakable determination to stick to something. Choose „hart“ for the effect of a measure and „eisern“ for firm resolve.

Der Verein hielt mit eiserner Disziplin an den Trainingsregeln fest.
The club stuck to the training rules with iron discipline.
Der Verein führte harte Trainingsregeln ein, die viele Mitglieder belasteten.
The club introduced strict training rules that put a strain on many members.