Impenetrable

Part of speech: adjective (a.)

1. Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious; as, an impenetrable shield. Highest woods impenetrable To star or sunlight. Milton.

2. Having the property of preventing any other substance from occupying the same space at the same time.

3. 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.