Harker could be a placename, or come from the anglo-saxon word meanng 'listener'. In the latter context it might be applied to a medieval watchman, or to a nosey parker.....
It's from a placename. There are two places in Normandy called Harcourt. Someone from there will have come over with William the Conk, bringing the name with them. It became a family name, with the pronunciation and spelling changing over the years.