Impenetrable Im*pen"e*tra*ble, a. [L. impenetrabilis; pref. im- not + penetrabilis penetrable: cf. F. imp['e]n['e]trable.] 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]

The Collaborative International Dictionary of English. 2000.