Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Impossible to capture or enter by force: an impregnable fortress.
  • adj. Difficult or impossible to attack, challenge, or refute with success: an impregnable argument.
  • adj. Capable of being impregnated.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. Too strong to be penetrated.
  • adj. Capable of being impregnated.

from the GNU version of the Collaborative International Dictionary of English

  • adj. Not to be stormed, or taken by assault; incapable of being subdued; able to resist attack; unconquerable
  • adj. Capable of being impregnated, as the egg of an animal, or the ovule of a plant.

from The Century Dictionary and Cyclopedia

  • Not pregnable; not to be taken or reduced by force: as, an impregnable fortress.
  • Not to be moved, shaken, or overcome; invincible: as, impregnable virtue.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • adj. incapable of being overcome, challenged or refuted
  • adj. immune to attack; incapable of being tampered with
  • adj. capable of conceiving

Etymologies

Middle English imprenable, from Old French : in-, not (from Latin in-; see in-1) + prenable, pregnauble, pregnable; see pregnable.
impregn(ate) + -able.
(American Heritage® Dictionary of the English Language, Fourth Edition)
Ultimately from im- ("not"), French prendre ("to take"), and -able ("able to be the object of an action"). (Wiktionary)
From impregnate +‎ -able, ultimately from Latin impraegnatus ("made pregnant"). (Wiktionary)

Examples

  • Charleston is a strongly fortified place and I think impregnable from the water side.

    Letter from Wright M. Carter to Young J. and Mollie Allen,May 6, 1861

  • Given the regular news reports of enterprise data lost through laptop theft, third party misfeasance, WEP hacking, loss of backup tapes, disgruntled employees, etc., it’s not like enterprise data is currently held in impregnable Kryptonite fortresses.

    Another Half-Dozen Half-Truths of the Cloud

  • It was this land side, containing the citadel and other works, which so impressed outsiders with the idea of impregnable strength.

    The Great Fortress : A chronicle of Louisbourg 1720-1760

  • You will storm that great fortress which men call impregnable -- you will storm it and you will vanquish it; and you will come home crowned with glory and honour!

    French and English A Story of the Struggle in America

  • I also knew that if we could by any means defend this doorway the murderers could get in nowhere else; for the palace is absolutely impregnable, that is, since the secret door by which Sorais had entered on that memorable night of attempted murder had, by Nyleptha's order, been closed up with masonry.

    Allan Quatermain

  • So he built an "impregnable" fortress atop the rock.

    Sigiriya

  • And never fear: the harmless, "impregnable" Iranians will be more than happy to aid their Shia religious brethren to victory in that battle.

    Archive 2005-12-01

  • Metcalfe said her department had put into place an "impregnable" security system which had been breached.

    ANC Daily News Briefing

  • The greatest victory of the last year was, of course, the successful breach on June 6, 1944, of the German "impregnable" seawall of Europe and the victorious sweep of the Allied forces through France and Belgium and Luxembourg -- almost to the Rhine itself.

    State of the Union Address (1790-2001)

  • Not that I subscribe to a view which you will often hear: that if Japan once gets the Phillipines, Singapore, Rangoon, and the Netherland Indies, she will be established in an "impregnable" position in the South West Pacific.

    The Next Few Weeks in the Far East

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Comments

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  • See unbearable.

    March 10, 2010

  • better yet, the two meanings of "inflammable"--that which isn't flammable; that which can be inflamed.

    May 21, 2009

  • I think there are two different prefixes which both look like "im". The first is "not", and the second is roughly "in".

    April 2, 2008

  • It seems awfully weird that impregnable and impregnate mean kind of opposite things. You'd think it would better be inimpregnable.

    April 2, 2008

  • (Mumm-Ra / "THE UNHOLY ALLIANCE")

    "SkyTomb is impregnable"
    -Chilla,"MUMM-RA LIVES - PART III"

    March 18, 2008

  • Contronymic in the sense: impossible to penetrate vs. penetrable, suseptible to impregnation.

    January 31, 2007