Many people with intellectual disabilities are very well aware of their disabilities and will know you’re making fun of them based on them. Even the ones that don’t know the definition of “retarded” will still hear the venom in your voice and your sneering laughter. If they’re so disabled that they can’t understand human speech, you’re still bound to piss off the people who are working tirelessly to take care of their family member.

Using “retarded” as an insult in general is degrading people with those disabilities, even if you’re not using it against someone with one. They are owed basic human dignity, just like everyone else. If you knew someone who was raped, would you call them a slut in reference to that?

If you must insult someone, say something based on a choice, like their attitude towards people below them socioeconomically or the way they add so much stuff to their coffee that the actual coffee flavor is gone.