Hi! Sarcasm is a way of insulting someone in a teasing fashion. Example: A girl is asked, by her boss, to clean her area. She picks up a shirt and leaves the rest of the shirts on the floor. The boss comes back to see how the girl was cleaning and says, " Ya', you did a really good job." He didn't mean she had done a good job because she didn't. The key thing to tell the child is, "don't use sarcasm, it hurts and doesn't help." Have a good day!
You could explain the word by telling the child that it is a nice way for grownups to tell others that something they do is not right or good. You can tell the child it's a way of being rude but in a nice way in a situation where you do not want to say bad things to the person.