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Well not exactly, but according to the American Heritage Dictionary of the English Language, impregnable means:

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:

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"?]

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Apologies if I seem to have answered my own question more or less; I had only the question in the title in mind when I started typing… –  ShreevatsaR Jan 29 '11 at 6:01
Yes, 'pregnable' is one of those words you never hear, like 'pervious', 'vincible', and 'gruntled'. :) –  Mark Maxham Jan 29 '11 at 6:02
Personally, I can't remember ever seeing "impregnable" used to mean "cannot be impregnated." It seems like a simple mistake though given its morphology. –  tankadillo Jan 29 '11 at 6:24
Amusingly, the HBO original series Game of Thrones recently punned on the source of your confusion in the May 15th episode (titled The Wolf and the Lion). Here's the dialogue: Tyrion Lannister [referring to a castle]: "The Eyrie. They say it's impregnable." His companion: "Give me 10 good men and some climbing spikes -- I'll impregnate the bitch." It's almost embarrassing to say how ecstatic I was in that I knew exactly why the joke worked, because of this question ;) –  Uticensis May 16 '11 at 5:34

1 Answer 1

up vote 33 down vote accepted

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.

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Oh wow, +1. I skimmed through the etymologies and didn't even look beyond the "in-" part; didn't realise the actual parts of the words were so different! –  ShreevatsaR Jan 29 '11 at 7:43
Me either - it's amazing that two very different words can end up sharing the same unusual shape! –  gpr Jan 29 '11 at 21:29
@gpr So basically impregnable means capable of being impregnated and too strong to be penetrated but the two meanings and the associated words don't have any relation? Right.? –  Kraken Jun 18 '13 at 18:48
No, impregnable only means too strong to be penetrated; impregnatable means capable of being impregnated. –  gpr Jun 19 '13 at 23:27

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