ping' refers to the act of making a print
from less than the whole area of a negative
, (at least, it used to - see below).
Before the advent of digital photography, the photographer/printer usually reviewed a contact print and then decided how much cropping is needed to remove unwanted parts of the image or to enhance the impact of the final print. This was achieved by use of a photographic enlarger. Digital photography has made the whole process of image selection and enhancement a lot easier, of course.