1. Quality of being impenetrable.

2. (Physics)

Defn: That property in virtue of which two portions of matter can not at the same time occupy the same portion of space.

3. Insusceptibility of intellectual or emotional impression; obtuseness; stupidity; coldness.

IMPENETRABLE Im*pen"e*tra*ble, a. Etym: [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. Highest woods impenetrable To star or sunlight. Milton.

2. (Physics)

Defn: 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.

IMPENETRABLENESS
Im*pen"e*tra*ble*ness, n.