How about “blind” or “deaf”?
A good kasha. I would differentiate by pointing out that mentally retarded people were looked down upon more than deaf or blind people, so it’s more comparable to a racial epithet.
Also, you didn’t address the chiluk I made, in terms of a secondary meaning.
You’re right, I kind of missed it.
Now that I look at it, your chiluk (and I assume you’re referring to the fatso comparison) would be true if people weren’t insulted when being called fatso. This is probably worse, though, because I imagine that someone having a developmentally disabled child would be whole lot more sensitive about it than if they had an overweight child.
All that having been said, whatever the reason, I think it’s considered PC to avoid the term, and considered somewhat crude to use it.