This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Impossible to penetrate or enter: an impenetrable fortress.

adj. Impossible to understand; incomprehensible: impenetrable jargon.

adj. Impervious to sentiment or argument: an impenetrable heart.

adj. Not penetrable.

adj. Incomprehensible; inscrutable.

adj. Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious.

adj. Having the property of preventing any other substance from occupying the same space at the same time.

adj. Inaccessible, as to knowledge, reason, sympathy, etc.; unimpressible; not to be moved by arguments or motives.

Incapable of being penetrated; not penetrable, in any sense of that word.

Specifically, in physics, having the property of preventing any other substance from occupying the same place at the same time.

adj. not admitting of penetration or passage into or through

adj. permitting little if any light to pass through because of denseness of matter

adj. impossible to understand

"Even this strong man shrank from this idea and showed a very natural recoil as his glances flew about the ill-omened room and finally rested on the fireside over which so repellent a mystery hung in impenetrable shadow."

thick    dense    uncomprehensible    incomprehensible   

penetrable   

impervious    unimpressible    unintelligible   

dense    opaque    impassable    impregnable    impervious    unbroken    leafy    murky    sombre    icy