adamantine

Meanings

1. 1
   adj

   impenetrably hard or unbreakable; resembling diamond

   His adamantine resolve never wavered under pressure.

From Latin adamantinus, from Ancient Greek adamantinos (ἀδαμάντινος, 'of the hardest metal'), from adamas ('unconquerable, hardest substance'), from a- ('not') + damaō ('to subdue').

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