emphasis If you describe something such as a book or a theory as impenetrable, you are emphasizing that it is impossible or very difficult to understand. His books are notoriously impenetrable. = incomprehensible + impenetrably im·pen·etrably impenetrably detailed reports on product sales. = incomprehensibly
If you describe something such as a barrier or a forest as impenetrable, you mean that it is impossible or very difficult to get through. the Caucasus range, an almost impenetrable barrier between Europe and Asia
impossible to understand; "impenetrable jargon" not admitting of penetration or passage into or through; "an impenetrable fortress"; "impenetrable rain forests