If you say that a person or group is impregnable, or their position is impregnable, you think they cannot be defeated by anyone. The Bundesbank's seemingly impregnable position has begun to weaken. = unassailable
If you describe a building or other place as impregnable, you mean that it cannot be broken into or captured. The old Dutch fort with its thick high walls looks virtually impregnable = impenetrable
Not to be stormed, or taken by assault; incapable of being subdued; able to resist attack; unconquerable; as, an impregnable fortress; impregnable virtue
\im-PREG-nuh-buhl\, adjective: 1 Not capable of being stormed or taken by assault; unconquerable; as, an impregnable fortress 2 Difficult or impossible to overcome or refute successfully; beyond question or criticism; as, an impregnable argument