If I ever knew, I had forgotten that impregnable “is actually a Middle English malapropism for imprenable,” as Mark Liberman explains in this Log post, quoting the OED:
Etymology: Corrupted < impreignable, imprenable, < French imprenable, < im- (im- prefix2) + prenable able to be taken, < pren-, stem of prendre to take. The g was evidently in imitation of the g mute in reign, deign, and the like, though it appears to have sometimes led in 16th cent. to the pronunciation /nj/
There is much discussion of etymological and semantic confusion, and some of baseball (the post takes off from a quote that mentions “the impregnable glove of Adrian Beltre at third base”). Check it out.
I should mention that I am experiencing severe computer problems at the moment (yesterday I foolishly tried to download a Windows Vista service pack that apparently didn’t download completely and left the computer hung up betwixt and between, unable to load Windows); while the situation continues, I will try to do necessary weeding and upkeep but will not be commenting and posting as often—I don’t like tying up my wife’s computer any more than I can help.