Earlier quoted context omitted.
Let's compile a list of things it's not as offensive as.
My brother is a retard. Your serve
Well my brother is not autistic. My brother has issues with understanding language and expressing himself. That does make it had for him to relate to ordinary people. But it doesn't mean that he has issues with socialization and emotional attachment. He desperately wants to fit in with people and have meaningful relationships with people.
As such I find the term Autistic, which assumes that he doesn't care about human emotional attachments. And assumes that he's somehow 'sick' to be extremely insulting.
I'd much rather people use the term retard because at least that just generally says that a person has one or more learning deficits and picks certain tasks slower than normal.
And frankly you're never going to get off the euphemism treadmill. That said for the reasons above, the concept of 'retard' is better than autistic by a long shot.