A Coal Mine is an insult from a friend or family member that uses an embarassing or deep secret of your's to hurt or embarass you. Usually used in a large crowd (where it has more exposure to others.) or to use against you.
1. Bill doesn't have a girlfriend anymore because his brother Coal Mined him in front of her.
2. "Hey Bob, get me a drink."
"Why should I?"
"Hmm... Perhaps you've told me something I can Coal Mine you with."