Discussion Groups / English Usage / January 2007

I think there is a certain amount of confusion here. It is isn't really
an "im-" prefix but (in each of the two words) an "in-" prefix in which
the m has been modified by the p that follows into a more easily
pronounced n. So if one allowed a negating prefix to be added to
"impregnable" it would be "in-" (or more likely "un-"), giving
"inimpregnable" or "unimpregnable". However, you are right that there
are two different prefixes with two different meanings that are written
in exactly the same way. It's just one of those things: English wasn't
worked out by a committee, but just evolved over the centuries.

Neither "inimpregnable" nor "unimpregnable" is in the SOED, but
"unimpregnable", at least, seems a perfectly possible word and one that
would be understood if used, especialled as "unimpregnated" does exist
in the dictionary.

It is certainly not the case that "the two negative prefixes cancel
each other out to give you the word 'pregnable' ", because there is
only one negative prefix. As you point out yourself, the "im-" in
"impregnable" is not negative. Even if it were, it would be very rash
to think that two negative prefixes would cancel one another.

Finally, why "un-" rather than "in-"? I think all the negating words
made by prefixing a word beginning "in-" or "im-" (like "unimpressed")
use "un-" or "dis-" or "anti-" rather than "in-", and I would guess
that it is for the obvious reason that "inin-" would sound nasty.

athel

Yes.  According to M-W on line, "impregnable" used to be "imprenable"
(untakeable).  I didn't know that.  M-W doesn't say where the g came
from.

No.  First, it would be "inimpregnable", as the n in in- changes to an
m only before b, m, or, p (and to an l before l and to an r before
r--unless you're finding this immaterial, illegitimate, and
irrelevant).  Second, the word I know is "unimpregnable".  For some
reason it isn't in M-W or AHD on line.  It gets 3,020 Google hits,
which is a small number, but a lot more than "inimpregnable" with 14.

That would be silly.

What would it mean?  D&C?

Others have answered your questions about the form of these words.  I
would like to add a few details, some of which may seem a little
speculative.   The opposite of "impregnable" (meaning "able to be
taken") is indeed made by removing the negative prefix: "pregnable".

There is another possible adjective in the same form as "impregnable"
meaning "able to be impregnated", a perfectly regular formation from
the verb.  This is not in the dictionaries, having been skunked to the
max by the other (mis-spelled) usage.  The word "unimpregnable" is not
in the dictionaries either, for the same reason; but it would be the
negative of the unused adjective: "incapable of being impregnated".

You can't impregnate someone who's impregnable.