I've always thought that word meant "capable of being penetrated". After all, a hapless female doesn't get PREGNATED, does she? She gets IMpregnated. OK, sometimes she does it with malice aforethought. Which isn't the main point, and you know it.

So why do I hear people using the word "impregnable", penetrable, obviously intending the opposite? Am I completely out of sync with current reality?

Kinda like "impatient" or "imperfection", praps?

Yeah, maybe a better question would be why the bloody hell should the positive form of a descriptive word start with a negative sounding prefix. Cause it cain't start with a suffix, for one thing. Also, because "im" doesn't really meant "not" in this case, but "into". So it seems to me.

Or, to unscrew the inscrutible.

Another trip down AHD Lane:

Turns out it means both able to resist capture or entry by force (from in-, not + prenable, PREGNABLE (from Old French prendre, to take, capture, from Latin prehendere)) and able to be impregnated (from IMPREGNATE meaning to make pregnant (from in- in + praegnas, PREGNANT))

Well, shut my mouth.

Ha, thousands have tried. Just ask Anna S.

a hapless female doesn't get PREGNATED, does she?

Not lately anyway, if at all. Onliest citation for PREGNATE in the OED is in reference to soil becoming fertile and dates to 1706.

...thousands have tried.

Must.... resist....temptation.

In that case, shouldn't it be
inimpregnable?
P.S. Is my first line correct? If not,please, how should I write?

Your question was worded impeccably, emanuela. In the one case inimpregnable would be a double negative. Make of that what you will. In the other case it would mean incapable of being made pregnant.