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.

Full definition of impregnable

Adjective

1. (of a fortress, wall, etc., also used figuratively) Too strong to be penetrated.
• SouthThe man's affection remains wholly unconcerned and impregnable.
• 2011, October 2, Jonathan Jurejko, Bolton 1 - 5 Chelsea, 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.

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

Adjective

1. Capable of being impregnated.Following treatment, twenty percent of previously infertile females were impregnable.Application of the compound rendered the non-porous surface impregnable.