It's an archaic medical term to refer to somebody with an *extremely* low IQ. I don't know anyone who takes it seriously any more and it's highly offensive in that context.

But the common usage has very much become "Somebody who keeps doing stupid stuff."

It demonstrates the problem. Most people who use the word have no idea it means that, don't mean that...they mean somebody who drives drunk late at night without a seatbelt, ya know.

So, what do we do about those words? If you use the obvious synonym of "fool," then that sounds archaic because people don't use it as much. (Archaic would be using "fool" to refer to that person who rambles on at the meeting...and huh, apparently it comes from the word for "bellows")