When someone says something that you have previously said and tries to pass it off as their own. You either hear about it from whoever they said it to or you overhear them saying it. Can be extremely irritating, especially if you're the sort of person who dislikes making a scene and doesn't want to call them on it.
Tom: Did you know that (insert interesting fact here)?
Dick: Wow, really? That's so interesting...
Harry: Did you know that (insert same interesting fact here)
Tom: Yeah, did I tell you that?
Harry: No, Dick did. He's so interesting.
Tom: MOTHERFUCKER!! That's conversational plagarism right there...