The man or woman who routinely steamrolls their way into a private conversation and makes it their own, usually stealing one conversation partner and leaving the other feeling jilted.
Common incident in staff rooms, classrooms, etc., where interesting conversations are few and far between.
Esmeralda: "Oh you're so funny Rufus, I would be so unhappy without you to talk to."
Rufus: "So I was wondering, well if you're not too busy tonight, maybe we could-"
Jean-Claude: (pokes Esmerelda) "Traffic was bad this morning..."
Rufus:(looking totally cast aside)"That fucking Jean-Claude... what a conversation thief!"