adjective
not able to be penetrated or pierced; impossible to pass through or enter
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The fortress was surrounded by an impenetrable wall, making it nearly impossible to breach.
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I tried to open the lock, but it seems impenetrable. I can't get it to budge.
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The security on that website is so impenetrable, it's like Fort Knox!
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Her stoic demeanor made her seem impenetrable, but deep down she was very vulnerable.