Thing To Be Today (or not): impregnable.

 What does this word mean?

Is something impregnable something no one can enter, as in an impregnable castle? 

Krak des Chevaliers, Syria, Photo by Bernard Gagnon

Or the same thing in a figurative sense, as in impregnable to flattery or bribery?

Or is it some animal which can be made pregnant?

pregnant lioness. Photo by Robin Hutton

Well, it's both, of course. This makes the word a contranym (a word that can mean itself or its own opposite).

I tell myself that one day I'll get the hang of this flipping English language...

...but there's not much sign of it so far.

Thing To Be (or not): impregnable. The word meaning not-able-to-enter dates from the 1400s and comes from the Old French imprenable, from prendre, to take. The word meaning able-to-become-pregnant dates from the 1600s and comes from the Latin praegnans, pregnant. The im- in the first case means not and in the second the im- means into, towards, or within.