Is it uncommon elsewhere? To my daughter my dad was “Pop Pop” and my wife’s dad is “Pappy.”
i have no heard anyone else referred to as PAP or any derivative thereof.
Pop Pop is a northeastern name for a grandfather. I had never heard it in my life (or Mom Mom) until we lived in PA. I even have a neighbor from PA who is “Mom Mom” and I smile when I think about how it is so commonplace in that area and not at all known down here. We usually hear names like: Nana, Grammy, Meemaw (which I don’t care for), or Grandma. My Mother is Nana, and her husband is Papa. My husband’s mother is Grandma Mary. His dad and step mother are burning in hell so we don’t call them anything. ;^)