Well not exactly, but according to the American Heritage Dictionary of the English Language, impregnable means:
ADJECTIVE:
1. Impossible to capture or enter by force: an impregnable fortress.
2. Difficult or impossible to attack, challenge, or refute with success: an impregnable argument.
Or, according to Wiktionary's short definition:
(Of a fortress, wall, etc.) Too strong to be penetrated.
On the other hand, the meanings of impregnate, in addition to "make pregnant", include
3. To fill throughout; saturate: a cotton wad that was impregnated with ether.
4. To permeate or imbue: impregnate a speech with optimism. See Synonyms at charge.2. to make a substance such as a liquid spread all the way through something: a pad impregnated with natural oils
1. a : to cause to be filled, imbued, permeated, or saturated
b : to permeate thoroughly
which, at a stretch, are closer to the "penetrate" meaning. Hence the question in the title.
Why is this? I guess it's because, as with inflammable and flammable, the two words come from different meanings of the in- prefix; impregnable from a "negation" meaning and impregnate from an "into" meaning. (And indeed, the AHD gives etymology with in-1 for impregnable, and in-2 for impregnate.) Is this right?
Some dictionaries also list a meaning for impregnable that come from impregnate and seem the opposite (in a loose sense) of the meaning above: the same AHD gives, as its second definition for impregnable,
ADJECTIVE:
Capable of being impregnated.
and some Hutchinson's Dictionary of Difficult Words gives:
impregnable
a. able to withstand attack; capable of being fertilized; able to become or be made pregnant.
Are these "capable of being impregnated" meanings for impregnable common?
[Finally, if one does want to express the 'easily penetrable' meaning, as in "This badly designed bulletproof jacket is easily penetrable", can one use "pregnable"? "non-impregnable"? Is there risk of confusion that "pregnable" may mean "can be made pregnant"?]
Best Answer
The two words have very different etymologies.
Impregnate comes from Latin impraegnare, which means 'to be imbued or saturated with'.
Impregnable comes from Middle French imprenable, itself derived from Latin prehendere, which means 'to take, grasp'.
That they have come to look so similar in English today is just coincidence.