Definitions

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

Etymologies

From Latin impenetrabilis. (Wiktionary)

Examples

  • Announcing his response to a formal consultation, Cable outlined plans for greater diversity on boards – including requiring boards to contain two people who had never had seats in boardrooms before – and to make remuneration reports, which he described as "impenetrable", easier to understand.

    Vince Cable hands shareholders power to tackle executive pay

  • It was not in impenetrable shadow as the other objects in the yard were, but had a dismal light about it, like a bad lobster in a dark cellar.

    A Christmas Carol by Charles Dickens, Stave 1 Marley’s Ghost | Solar Flare: Science Fiction News

  • Even this strong man shrank from this idea and showed a very natural recoil as his glances flew about the ill-omened room and finally rested on the fireside over which so repellent a mystery hung in impenetrable shadow.

    The Filigree Ball

  • Like a cloud, like one of many that were spread in impenetrable woof over the sky, which, when the shepherd north has driven its companions "to drink Antipodean noon," fades and dissolves in the clear ether -- Such were we!

    III.8

  • From that moment the primate of Egypt disappeared from the eyes of his enemies, and remained above six years concealed in impenetrable obscurity.

    The History of the Decline and Fall of the Roman Empire

  • The only reason to write legislation in impenetrable legalize is a desire to hide what they’re doing from their constituents and one another (and, I suppose, tradition, upheld to preserve their ability to hide what they’re doing in the future.

    Matthew Yglesias » The Pointlessness of Reading Bills

  • The supreme confidence of the Romney organization - one advisor called his Florida firewall "impenetrable" - has been shaken in recent days by the former Massachusetts governor's repeated stumbles over his personal taxes.

    News - latimes.com

  • No helmet prevented a head struck by the edges of the Crusaders 'swords from being wounded; and they found the stitching of their so-called impenetrable cuirasses too fragile.

    The Deeds of God Through the Franks

  • ACE said it had consulted widely and feedback had included complaints that the current funding process was "impenetrable" and "a closed shop".

    Arts groups face dramatic funding shakeup

  • Philip Alston of the UN Human Rights Council charges that secret "international intelligence services" allied with local militias are killing Afghan civilians and then hiding behind an "impenetrable" wall of bureaucracy.

    Conn Hallinan: Shadow Wars

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  • "He glazed over. Drifted a moment. Went through something in his impenetrable interior ... "Actually I do know this is hard for you. I'm sorry. I'm not using my imagination. That was my New Year's resolution, you know. Work on standing in the other fellow's shoes. That and to read one poem every day." From "The Last Werewolf" by Glen Duncan.

    March 23, 2012