Rara, What's the "A" word mean?

I had this conversation with Boston the other day. He sprang it on me as I was folding clothes. It went something like this:

Boston: "Rara, what does the "A" word mean?
Me: "Huh? What "A" word?"
Boston: "You know, the "A" word."
Me: "Where did you hear that?"
Boston: "At McDonald's, when we were playing, there was boy who said, 'she said the "S" word and the "A" word.' " Then he says, "I know what the "S" word is but I don't know what the "A" word means?"
Me, semi-mortified: "You know what the "S" word means?!"
Boston: "Yes, it means stupid."
Me, breathing a HUGE sigh of relief that my son's innocence has been preserved for at least a little while longer: "Yea, stupid is a pretty bad word, isn't it?" :-)