Before the r-word, there was this term that came from the French word for Christian!
Over the years, there have been plenty of offensive names for those with mental disabilities.
With increased understanding and sensitivity to these issues, these names have thankfully diminished. The first term used to describe the mentally handicapped was actually quite kind though.
"Cretin" is considered a term of abuse today. However, when it was first used, it was not seen that way. It comes from a French word for Christian.
It conveyed that those with mental disabilities were still human and still Christian. They deserved to be treated with dignity and were thought to be incapable of sin. Thus, they were Christ-like.