Impenetrable (adjective)

1. Not able to be entered, passed through, or understood; impassable.
2. Not able to be penetrated or understood; incomprehensible.
3. Not able to be affected or influenced; resistant.

Origin:

From the latin word "impenetrabilis" meaning "not able to be penetrated".

Examples:

1. The jungle was impenetrable, and the explorer had to turn back.
2. The book's author had an impenetrable writing style that was difficult to understand.
3. The impenetrable barrier of ice was impossible to break through.
4. The security system was impenetrable, making it impossible for intruders to enter.
5. The impenetrable defenses of the fortress made it impossible for the enemy to conquer.