1.
If you describe something such as a barrier or a forest as ~, you mean that it is impossible or very difficult to get through.
...the Caucasus range, an almost ~ barrier between Europe and Asia.
ADJ: usu ADJ n
2.
If you describe something such as a book or a theory as ~, you are emphasizing that it is impossible or very difficult to understand.
His books are notoriously ~.
= incomprehensible
ADJ emphasis
impenetrably
...impenetrably detailed reports on product sales.
= incomprehensibly
ADV: ADV adj