Pronunciation: (im-pen'i-tru-bul), [key]
1. not penetrable; that cannot be penetrated, pierced, entered, etc.
2. inaccessible to ideas, influences, etc.
3. incapable of being understood; inscrutable; unfathomable: an impenetrable mystery.
4. Physics.possessing impenetrability.adv
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.