I think this is a perfectly legitimate question.
If someone doesn't see something that is right in front of their face, I might say, "Are you blind?" as blind means someone who has trouble seeing or can't see at all.
If someone is having trouble grasping an extremely elementary concept, I might say, "Are you retarded?" as retarded means someone who has cognitive disabilities and therefore will not get concepts that other people would understand.
There are people that will say either of the above is
ablist and offensive. I get that. I know there are people in the world who feel that way. If someone were to call me on my use of either of those terms, I would apologize and not use them in that person's presence. I wouldn't argue that "retarded" is no longer the preferred term (developmentally or intellectually disabled is) so it is now completely divorced from its previous meaning and ok to use. I wouldn't say I took a survey of 20 people on my street and none of us had ever seen a blind person so it is not offensive. I would accept that using those words had consequences and move the fuck on.
"Crazy" is the same. If someone objected to me using the word, I wouldn't, even if I didn't personally agree with their reasoning. That's just the choice I would make. It's not a hardship to refrain from using a certain term out of politeness.