Noah Webster's Dictionary 1. (a.) Not to be taken by assault; incapable of being subdued; unconquerable; as, an impregnable fortress; impregnable virtue. 2. (a.) Capable of being impregnated, as the egg of an animal, or the ovule of a plant.

Multi-Version Concordance Impregnable (1 Occurrence) Isaiah 33:16 He will have a place on high: he will be safely shut in by the high rocks: his bread will be given to him; his waters will be certain. (See NAS)