Real Dictionary

impregnable
adjective: If you describe a building or other place as impregnable, you mean that it cannot be broken into or captured.
adjective: If you say that a person or group is impregnable, or their position is impregnable, you think they cannot be defeated by anyone.

impregnable in American English 1
not capable of being captured or entered by force
adjective: strong enough to resist or withstand attack; not to be taken by force, unconquerable

impregnable in American English 2
that can be impregnated
adjective: susceptible to impregnation, as an egg

impregnable in British English 1
adjective: unable to be broken into or taken by force

impregnable in British English 2
adjective: able to be impregnated; fertile