Every word that was used to refer to someone with some form of mental difference has been turned into an insult. At a certain point you need to just get over it.
Dumb, idiot, imbecile, slow, retard, etc were all originally used to refer to people with legitimate mental health issues and have been turned into insults and are now considered rude to say.
It doesn't matter what word you come up with to try to "fix" this is going to happen.