This idiom literally means to drive a nail into the chest. The actual definition means to hurt someone emotionally. In other words, to break someone's heart. The idiom describes the intense feeling one gets when something is said or done that causes a person to feel as if someone is backstabbing him/her in the heart. You can use this phrase when you're talking about a heartache, breaking up, or when someone tells you and/or another person something very direct, harsh, and unfiltered.