from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Impossible to penetrate or enter: an impenetrable fortress.
- adj. Impossible to understand; incomprehensible: impenetrable jargon.
- adj. Impervious to sentiment or argument: an impenetrable heart.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Not penetrable.
- adj. Incomprehensible; inscrutable.
from the GNU version of the Collaborative International Dictionary of English
- adj. Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious.
- adj. Having the property of preventing any other substance from occupying the same space at the same time.
- adj. Inaccessible, as to knowledge, reason, sympathy, etc.; unimpressible; not to be moved by arguments or motives.
from The Century Dictionary and Cyclopedia
- Incapable of being penetrated; not penetrable, in any sense of that word.
- Specifically, in physics, having the property of preventing any other substance from occupying the same place at the same time.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. not admitting of penetration or passage into or through
- adj. permitting little if any light to pass through because of denseness of matter
- adj. impossible to understand
From Latin impenetrabilis. (Wiktionary)