impenetrable

a. [ L. impenetrabilis; pref. im- not + penetrabilis penetrable: cf. F. impénétrable. ] 1. Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious; as, an impenetrable shield. [ 1913 Webster ]

Highest woods impenetrable
To star or sunlight. Milton. [ 1913 Webster ]

2. (Physics) Having the property of preventing any other substance from occupying the same space at the same time. [ 1913 Webster ]

3. Inaccessible, as to knowledge, reason, sympathy, etc.; unimpressible; not to be moved by arguments or motives; as, an impenetrable mind, or heart. [ 1913 Webster ]

They will be credulous in all affairs of life, but impenetrable by a sermon of the gospel. Jer. Taylor. [ 1913 Webster ]