"They's Heathcliff and a woman, yonder, under t'Nab"

Park Nab
Creative Commons Attribution Share AlikePark Nab - Credit: Mick Garratt
A nab is the part of a hill or rock formation that projects outwards. It would provide some sort of shelter, very welcome on a cold, windy day on the moors.

It would have been the perfect refuge for Cathy and Heathcliff.