Most commonly seen in fanfiction this is an action used to deter bullies and haters in which a commenter will express support for a content creator or poster while at the same time acknowledging the bully/hater without giving them satisfaction. This is usually done by expressing your support in the reply section for the bully/haters' comment or very shortly apologizing for their actions.
"Have you ever used the classy fuck you? I've seen it work pretty well."