I'm 33 years old and grew up in the southeast US. Maybe it's a generational or regional thing or maybe I just grew up around some genuinely hateful people, but calling people that word was very commonly paired with other things you can't just explain away. Terms like "sped" (short for special education which was the term for the special courses people with disabilities went through in school), "short bus" (referring to the school buses that were shorter and used for children with special needs), and others were used. Actions like mimicking the voice impairments or movement impairments of people with disabilities was common. All of these things were, just like the one word this thread is about, just like people are saying it's used today, "just" meant to insult people for being stupid.