A resident student or researcher at the University of Cambridge UK.
A native (IE born in or moved to while in infancy) Cambridge resident. As Londoner is the collective noun for London residents, Cantabridgian applies for Cambridge (UK).
Person 2: Have you ever been there?
Person 1: No, but everything remotely intelligent and hip about Boston is actually in Cambridge.
Person 2: Oh right, your a Cantabridgian.
Occasionally shortened to "Cantab." Obviously, this abbreviation is very easy to joke about.
Considering the fact that they attend the aforementioned university, this person is pretty much going to be a genius.