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From Biology-Online Dictionary
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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. (Science: physics) 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)

Origin: L. Impenetrabilis; pref. Im- not _ penetrabilis penetrable: cf. F. Impenetrable.