What does "impenetrable" mean?

Impenetrable: (Physics) Having the property of preventing any other substance from occupying the same space at the same time.

Additional senses

  1. 2.Inaccessible, as to knowledge, reason, sympathy, etc.; unimpressible; not to be moved by arguments or motives; as, an impenetrable mind, or heart. They will be credulous in all affairs of life, but impenetrable by a sermon of the gospel. Jer. Taylor.

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