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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"?]

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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
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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
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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
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Just FTR (having deleted long and winding comments about this) note that the phrase "which, at a stretch, are closer to the "penetrate" meaning" is, simply, quite wrong. The excellent definition of impregnate (ie soak, imbue, stain, etc) given as a quoted section immediately before this phrase simply has not the slightest hint or suggestion of "rip, tear, penetrate, break in to". Cheers –  Joe Blow Jun 9 at 3:37

2 Answers 2

up vote 38 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
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No, impregnable only means too strong to be penetrated; impregnatable means capable of being impregnated. –  gpr Jun 19 '13 at 23:27
    
I take it, impregnable should actually have been imprehensible or something like that. –  Andriy M Jun 8 at 7:53

Just for the record, since this question has caused some confusion. Future readers (particularly English learners) may find this useful. The word

impregnate

simply mens "soak". So, if you're an engineer, or you make shoes, you use it constantly, along with words like "weld" or "truss" or "staple".

"OMG, the dye is impregnating the steel plate." "This impregnating machine is crap dude." "How the hell are we going to get the glue to impregnate this weird nylon stuff?"

Here are some impregnating machines,

http://www.menzel-maschinenbau.de/en/products/impregnating-machine-grid-fabric-m2224-05/

enter image description here

http://www.vanwees.nl/products-services/impregnation-machines/ http://www.godfreywing.com/vacuum-impregnation/equipment

Here's some impregnated sleeving (beautiful isn't it?)

http://www.detakta.de/en/glasfibre-sleevings/glassfibre-sleeving-impregnated-with-silicone.html http://www.atkinsandpearce.com/coated-insulation-solutions/suflex/acrylic-sleeving/acryflex-vpi/

Some impregnation substances,

http://www.isomat.eu/Waterproofing-of-walls-by-water-repellent-impregnation.html https://www.youtube.com/watch?v=skST_iPIj1U

Here's some leather impregnation tips,

https://www.youtube.com/watch?v=skST_iPIj1U http://www.schuhdealer.com/shoeblog/schuhe-richtig-impragnieren/

enter image description here

Here's a whole pile of impregnating stuff and impregnated stuff,

http://www.alibaba.com/trade/search?SearchText=impregnating http://www.isve.com/en/impregnation-treatment-and-coating-of-wood-in-autoclave-using-double-vacuum-system

So, impregnate -- soak.

Now, consider the word "impregnable". It has utterly no relationship, in any way, meaningwise or etymologically, to "impregnate". But "impregnable" and "impregnate" (coincidentally) look similar. Further, as the OP points out, various variations of these words are also confusingly similar. (I'm sure you could easily find thousands of examples of people using these words the wrong way, mixing-up the two words: which is an absolutely commonplace phenomenon in English usage.)

Now. I actually do not know what the questions is asking as there is no decisive "question asking" part to the question. But the general tenor of the question seems to be "what's the deal with these two words."

(1) Note that gpr has perfectly explained that there is no, zero, etymological connection.

(2) I suspect that, particularly for any English learners reading in the future, there is some confusion about the word "impregnate"/"soak". I have tried here to take some pains to show the everyday use of the very common normal word "impregnate"-ie-"soak" which has a spectacularly clear meaning (to wit ........... "soak" !) My point with this answer is that hopefully it will make it quite clear that "impregnate"/"soak" has no connection at all to issues of medieval warfare, breaking into castles, etc.

(3) I think the overwhelming takeaway point is that it is utterly common in English that two words "happen to sound or look similar" but in fact have zero connection in any way (meaning, use, or etymology). This is part of the general "English is a spectacular mess" phenomenon of all English spelling and usage. Indeed note that it's a common feature on this site that a, perhaps new English learner, will ask questions about two coincidentally similar words ...... and the answer is nothing more than "oh that's a coincidence, the words are 1000 years apart in etymology and have no connection at all in meaning." This is completely commonplace in English.

(4) A final point is that "sometimes the in- prefix means 'into', and sometimes it means 'negation'." (And additionally im- and in- are simply alternate spellings of the same prefix.) But this observation is so commonplace on this site and ELL that it's barely worth mentioning. (The absolutely obvious joke based on the two meanings of the 'in-' prefix .. "So, is 'duce' the opposite of 'induce' ?! .. hahahahah!" must have been made 100 times on this site; often English learners will ask it for real, not as a joke.)

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I would definitely not call the medical/veterinary sense “obscure” by any means. It is perfectly commonplace. I’m fairly sure I’ve used that sense a lot more than the wood/leather/fabric sense (where I think I’d be more likely to just use the more generic proof anyway). –  Janus Bahs Jacquet Jun 10 at 10:46
    
Fine. The medical use is even more irrelevant and more of a "sheer coincidence". You've taken a tower of usefulness and logic and added a shingle, good one. Regarding "proof", that's great, I'll see you at the next medieval revivalist meet :) So, you'll be quick to point out that "impregnable" is a rarely used word. –  Joe Blow Jun 10 at 13:13
    
The question has not caused any confusion. –  Doop Jun 12 at 15:10

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