It means both EXPERT and AUTHORITY (in the sense of She is an authority on [whatever the field is]). It's not just an expert but one who is consulted, at least in the usage I see. That is, I am an expert in my field but nobody asks me about it so I don't consider myself a maven. My dictionary doesn't derive anything. Am also interested in how this came to mean what it seems to... looks Saxon rather than Norman to me...