Meaning of Retarded

Overheard on the schoolbus on the way to our field trip at Yaquina Head:

"The word retarded so does mean slow."
"No it doesn't."
"Yes, it does and we have parents to prove it."

This made me laugh internally for the rest of the way on the trip. What the 4th grader meant was that she had asked her parent what the word retarded meant.