Perfectly good word: "ignorant".
If someone's ignorant in regards to the history of Mesoamerican history, or nuclear physics, or hydromechanics, that doesn't mean they're irrational/insane towards it.
Ignorance has nothing to do with how the word "insane" has been typically used in Less Wrong. It's been typically used to refer to people whose minds don't properly handle information about reality and/or make counterproductive decisions even though they have the information they require to not do so.
Indeed if someone's merely ignorant, and that's why they don't make the optimal decisions, that's a vastly smaller problem than when they have all the required information and still make insane decisions.
Edit :Excellent suggestions in the comments. Two of them stood out for me:
We often use "insane" to describe people whose behaviour or beliefs are below the sanity waterline. But as most must would agree here, you cannot call someone insane with a straight face just because he happens to believe in magic.
I'm currently watching Future by Design, a documentary featuring Jacque Fresco and the Venus Project. Jacque came up with this word, "unsane", to describe people who basically, aren't rational because they haven't been exposed to the right ideas yet. Which would be different from "insane", which is more about irrevocably irrational people.
I like this word, because there isn't the tone of accusation we find in "insane". This neutrality makes it easier to say that we can do something about it. Insanity should be eradicated like vermin. Unsanty on the other hand can be fixed.
So, do you think this word, "Unsanity" might be worth using?