Sid Kaplan - printed Henri Cartier-Bresson. Hearing how Cartier-Bresson would photograph, it's a wonder the prints look as wonderful as they do.
Bob Carnie - one of our own members here on APUG. I have seen his prints, and all I can say is that attention to detail, as well as the content of the photograph, is amazing.
I think a master printer is able to understand what is necessary to show the content of the photograph effectively. His or her own, or somebody else's work. It is someone that can take all types of negatives, good and bad, and work with them until there is a result worthwhile. I do know that Bob Carnie has preferences of how the negatives are shot and processed, but I bet he can take a poor negative and make something good with it.
I agree, however, that it is a loose term, which probably is used differently by different people, based on their own understanding. It is more than a bit subjective what each of us would call masterful.