Rashi: It is a slingshot stone. It is to throw, and not to keep it.

Rashi: [The honor] does not last.

Rashi (from Chulin 133a): Teaching an improper Talmid is like throwing a rock at the idol Markulis. (He intends to destroy it. He does not realize that through this, he serves it!)

Malbim: This explains why not to answer a Kesil at length, honorably. Rather, answer him disgracefully. Just like it seems to honor a stone if he ties it in a slingshot, this is only to cast it at people and kill them. So is honoring a Kesil via answering with Chachmah. He casts it in a slingshot to kill also people on a pure path, for he transforms your answers to rocks of destruction and heresy.