Originally: the governor or chief officer of a town or borough; = "borough-reeve". After the Norman Conquest: a person equivalent in rank and office to a town mayor (now chiefly historical, but still current in a small number of boroughs, especially in south-west England and Wales).
Old English. From port + Old English gerēfa reeve. Compare post-classical Latin portegravius, portgravius, portgrefius, portgrevius, also portrevus. Old Icelandic port-greifi is from Old English.