If someone is self-confessed, they have said it themselves: they have this quality, and they are not hiding it. The older, sharper use is for something people look down on — a self-confessed liar, a self-confessed cheat. Though actually, writers stretch it all the time, often with a little smile: a self-confessed chocolate lover, a self-confessed nerd, a self-confessed night owl. Those are not crimes. The speaker is saying, “Yes, that’s me. I admit it.” It almost always sits in front of a noun (a self-confessed …). You will meet it in newspapers and chatty profiles more than in playground talk. It is not the same as confessing to the police. And if you call someone a self-confessed expert, it can sound as if you are teasing them, or as if they talked themselves up.