impregnable

From Middle French imprenable, im- (“not”), + prendre (“to take”) +-able (“able to be the object of an action”). Intrusive g added 16c on model of deign, reign.

impregnable (comparative more impregnable, superlative most impregnable)

1. (of a fortress, wall, etc., also used figuratively) Too strong to be penetrated.
   • South

     The man's affection remains wholly unconcerned and impregnable.

   • 2011 October 2, Jonathan Jurejko, “Bolton 1 - 5 Chelsea”, in BBC Sport‎:

     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.

• impregnability
• impregnableness
• impregnably

From impregnate +‎ -able, ultimately from Latin impraegnatus (“made pregnant”).

impregnable (comparative more impregnable, superlative most impregnable)

1. Capable of being impregnated.
   • 1979 November 1, “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 Behavior, volume 27, number 4, page 1098:

     The reproductive strategies of troop members, especially those of impregnable females, are suggested to influence patterns of range use.

• Webster's Third New International Dictionary (1966).