I hate how that word became pejorative, because it was used correctly. By the way, it’s still used in plumbing. Retard is a verb which means to slow, e.g. retard the flow. When you call a person who is developmentally disabled that, yes it’s rude, but it means their mental process is slow. The word was being used accurately. It’s just not nice to say.

I don’t think “window licker” was ever accurate, but for some reason it’s slightly more socially acceptable to say (or imply, e.g. “I will say this for him, his windows are always clean”).