J.B.
I didn't realize that "sexy" was a bad word, but it is used in a more adult context, so just tell him that kids his age aren't sexy. The word means "very attractive to a grown up." Nothing 2nd graders do or are involved in is "sexy." -Tell him that and that you don't want to hear of him or hear him using it again because it's inappropriate for his age group -then stop. As far as him copying other kids and saying it for a laugh -that's the most normal thing in the world for a child to do. Kids also LOVE to say words that they discover you think are bad or not for them to use. I would give him the explanation and let it lie. If you hear him using it, give him a look and tell him you don't want to hear it. The joy of the word will go away fairly quickly.