Off the top of my head: It's supposed to be the single unpardonable sin. Blaspheming against the Holy Spirit? Only there's mystery around what that involves exactly, so folks in power have used it to manipulate people by saying that x thing they don't like is the Very Special Unpardonable Thing.

After Googling: I thought it was something Paul said, but I tend to attribute things that irritate me to Paul. That was inaccurate, though. Apparently it was from the gospel of Matthew.