Noah Webster
Impenetrable
IMPEN'ETRABLE, adjective [Latin impenetrabilis; in and penetrabilis, from penetro, to penetrate.]
1. That cannot be penetrated or pierced; not admitting the passage of other bodies; as an impenetrable shield.
2. Not to be affected or moved; not admitting impressions on the mind. The hardened sinner remains impenetrable to the admonitions of the gospel.
3. Not to be entered by the sight; as impenetrable darkness. Hence,
4. Not to be entered and viewed by the eye of the intellect; as impenetrable obscurity or abstruseness.