Impregnable means so strong that it cannot be broken into. An impregnable fortress, an impregnable defence, thought impregnable.
Every fortress called impregnable eventually fell, which is the pattern of the word: it is a claim rather than a fact. It has nothing to do with pregnancy, despite the spelling, and comes from the French for not able to be taken.
1adjectiveimpregnableSo strong that it cannot be broken into.
“The team's defence looked impregnable, and they didn't let in a goal all season.”
impregnable is a quality.
The French behind it is to take. Impregnable is what cannot be taken, and it has nothing to do with pregnant.
hooks ontotake
Old French imprenable means not able to be taken — in, not, plus prendre, to take. The g crept in later.
Proto-Indo-European
*gʰed- — to find; to hold; to seize, take
Latin
prēndere
Old French
imprenable
Middle English
imprenable
English, today
impregnable
Built from
From Late Middle English imprenable, impregnable (“impossible to capture, impregnable”), from Old French imprenable (modern French imprenable (“impregnable”)), from im- (a variant of in- + prenable (“(military) of a building, position, etc.: takable”) (from prendre (“to take”) + -able). 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.
Etymology from Wiktionary, CC BY-SA