I would age appropriately explain sexy and that you would prefer her not to use it. That its not a bad word but it's not one little girls should use. I would also explain how each family is different and that language is inappropriate in your house. Explain how each parent has different rules.
My oldest is 12 and some of her friends over the years have been like this. It saddens me because your right.
Mom to Breille (cheese) 9-14-00 & Mackenzie (Mac) 12-31-10
I do know how to spell, my smartphone does not.