From Late Middle English imprenable, impregnable (“impossible to capture, impregnable”), from Old French imprenable (modern French imprenable (“impregnable”)), from im- (a variant of in- (prefix meaning ‘not’) + prenable (“(military) of a building, position, etc.: takable”) (from prendre (“to take”) + -able (suffix meaning ‘creating an effect or influence’)). Prendre is derived from Latin prēndere, present active infinitive of prēndō, a variant of prehendō (“to catch, lay hold of; to grasp; to grab, snatch; to seize, take”), ultimately from Proto-Indo-European *gʰed- (“to find; to hold; to seize, take”). The intrusive g in the English word was modelled after words like deign and reign.
1
Government, Military, Politics, War Of a fortress or other fortified place: able to withstand all attacks; impenetrable, inconquerable, unvanquishable.
Unerringly impelling this dead, impregnable, uninjurable wall, and this most buoyant thing within; there swims behind it all a mass of tremendous life, only to be adequately estimated as piled wood is—by the cord; and all obedient to one volition, as the smallest insect.1851 November 14, Herman Melville, “The Battering-ram”, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley, →OCLC, page 376:
Jurgis got up, wild with rage; but the door was shut and the great castle was dark and impregnable.1905 April–October, Upton Sinclair, chapter XXV, in The Jungle, New York, N.Y.: Doubleday, Page & Company, published 26 February 1906, →OCLC, page 292:
2
Figuratively Too strong to be defeated or overcome; invincible.
[A]s for the Friendſhip of the World; […] he may at laſt be no more able to get into the other's Heart, than he is to thruſt his Hand into a Pillar of Braſs. The Man's Affection, amidſt all theſe Kindneſſes done him, remaining wholly unconcerned, and impregnable; juſt like a Rock, which being plied continually by the Waves, ſtill throws them back again into the Boſom of the Sea that ſent them, but is not at all moved by any of them.1664; first published 1694, Robert South, “A Sermon Preached before the University, at Christ-Church, Oxon, 1664”, in Twelve Sermons Preached upon Several Occasions. […], volume II, London: […] J[ohn] H[eptinstall] for Thomas Bennet […], →OCLC, page 76:
And with Bolton suffering a wretched run of five straight home defeats – their worst run in 109 years – Chelsea fans would have been forgiven for expecting a comfortable win. But surely they did not anticipate the ease with which their team raced into an almost impregnable half-time lead.2011 October 2, Jonathan Jurejko, “Bolton 1 – 5 Chelsea”, in BBC Sport, archived from the original on 31 Dec 2021:
PIE word *h₁én From impregnate (verb) + -able (suffix meaning ‘able or fit to be done’ forming adjectives). Impregnate is either derived from impregnate (“pregnant”, adjective), or from its etymon Medieval Latin or Late Latin impraegnātus (“made pregnant”), past participle of impraegnō (“to make pregnant”), from Latin im- (a variant of in- (prefix meaning ‘in, inside, within’)) + praegnāre (“pregnant”) (from praegnāns, a variant of praegnās (“pregnant”), from prae- (prefix meaning ‘before; in front’) + *gnāscor (archaic), nāscor (“to be born; to grow, spring forth; to arise, proceed”) (ultimately from Proto-Indo-European *ǵenh₁- (“to beget; to give birth; to produce”)).
1
Capable of being impregnated; impregnatable.
The reproductive strategies of troop members, especially those of impregnable females, are suggested to influence patterns of range use.1979 November, Dennis R. Rasmussen, “Correlates of Patterns of Range Use of a Troop of Yellow Baboons (Papio cynocephalus). I. Sleeping Sites, Impregnable Females, Births, and Male Emigrations and Immigrations.”, in Animal Behaviour, volume 27, number 4, New York, N.Y.: Elsevier Science Pub. Co., →DOI, →ISSN, →OCLC, abstract, page 1098: