hard and impenetrable, viz. having the properties of adamant which was a name given to any very hard, tough minerals, such as diamond
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Uses:
supported by eight adamantine feet, each six yards high.
Jonathan Swift. Gulliver’s Travels (1726)
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Whitewash on the forehead hardens the brain into a state of obstinacy perhaps. Anyhow, with whitewash from the wall on my forehead, my obstinacy was adamantine.
Charles Dickens. Great Expectations (1861)
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he laid upon me the blame for his weakness’ mistakes, wanted to repair them with new outrages and yet more mortifying invectives; there was nothing he did not say to me, nothing he did not attempt, nothing his perfidious imagination, his adamantine character and the depravation of his manners did not lead him to undertake.
Marquis de Sade. Justine.
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When she was in fourth grade she’d been attacked by an older acquaintance, and this was common knowledge throughout the family (and by extension a sizable section of Paterson, Union City, and Teaneck), and surviving that urikán of pain, judgment, and bochinche had made her tougher than adamantine.
Junot Díaz. The Brief Wondrous Life of Oscar Wao (2007)