1. impossible to pass through or enter
2. impossible to understand or comprehend

Examples

• The jungle was so dense it seemed impenetrable.
• The fortress had impenetrable walls that no army could breach.
• His explanation was so technical it was impenetrable to most students.
• The mystery remained impenetrable despite years of investigation.
• She wore an impenetrable expression that revealed nothing.
• The ancient text was written in an impenetrable code.
• The fog created an impenetrable barrier across the highway.