So two things that makes this believable for me. One you grew into an adult so you weren't as naturally aware of the younger generation's lexicon.

And also you lived closer to the inception of the word. Its very possible that "Retard" met the specificity requirement in your time for a slur, and meant a very particular caricature in that time. Which, is what I think is need for a word to be slur. It needs two things Hate, and specificity towards a particular subset of humans. Think about all the other slurs, they very specific aspect to them when you think of the word, to the point you can't mistake it for another subset of humans. Which is my problem with Retard being a slur, even in the medical diagnoses it was an umbrella term. It wasn't like "Downs" that was bandied about in my childhood, and IMO is the real slur of my time for this subject.

So I think "Retard" may have just became generalized by the time it my generations lexicon, because I knew the definition as "Incredibly Stupid" before I had ever met a mentally disabled person. And when I had, trust me, I would have gotten my ass beat by my peers if I had even uttered "stupid" in there direction. Which, IMO, would have been the right call. Besides I would never have insulted Ronnie like that, kid was decent and kind.

Btw, this has been interesting to me, and I appreciate this discussion even though there are those that disagree. I hope you know I don't have any ill-will towards you, and very much enjoyed this.