What does "impervious" mean?

Impervious: Not pervious; not admitting of entrance or passage through; as, a substance impervious to water or air. This gulf impassable, impervious. Milton. The minds of these zealots were absolutely impervious. Macaulay. -- Im*per"vi*ous*ly, adv. -- Im*per"vi*ous*ness, n.

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