Noah Webster's Dictionary 1. (a.) Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious; as, an impenetrable shield. 2. (a.) Having the property of preventing any other substance from occupying the same space at the same time. 3. (a.) Inaccessible, as to knowledge, reason, sympathy, etc.; unimpressible; not to be moved by arguments or motives; as, an impenetrable mind, or heart.

Multi-Version Concordance Impenetrable (1 Occurrence) Jeremiah 46:23 They shall cut down her forest, saith Jehovah, though it be impenetrable; for they are more than the locusts, and are innumerable. (DBY RSV)