What is the "r" word, you ask? I don't know that I even need to define it because you'll know just what I mean if I use it in a quote or two:
"I can't believe I did that! I am such a retard!!"
"You are so retarded! Why do you have to act like such an idiot!"
I grew up knowing exactly what people meant when they used the "r" word: you are acting as badly as a person with mental retardation.
Guess what is the leading cause of mental retardation? Down syndrome.
Using our keen powers of syllogistic logic we can then say that if someone does something really stupid then it's appropriate among most people (even well-educated people I know) to compare them to a person with Down syndrome. If someone does something really stupid, it's appropriate to say that they are acting like Emma. Or Vera. Or Ella. Only a person with Down syndrome would do something that stupid.
I hear this a lot--the quotes above are verbatim. So to those people, and others like them, I have one final comment, adapted from a much harsher statement:
You are not retarded. You are a thoughtless jerk. My daughters are retarded and they would never do that.