One who tends to give too much information while telling a story. Often leads to other stories that have nothing to do with the original topic. The person then, at some point, loops back around to the original story and forgot what he was saying.
When spotted out to lunch with a woman, whom was not his wife, John went into over explainer mode.
John: Oh...hey guys, this is my niece, Mary. I just met her for lunch here because my wife asked me to give her something. Oh I know this place is kind of out of the way but we met here because she'd never been here before and she just graduated and uh, I normally don't go to lunch with other women but...