When I watched the video it was hard to believe that anyone would call JFR, "Pappy." But Knorr called him "Pappy." I remember at Bethel, you sometimes heard Knorr referred to as "Pappy Knorr." I can't see how anyone would give either of them such a nickname based on their natural personalities. Seems like a nickname they wanted to be called.
Interestingly, some Catholics call the Pope "papa," eg "Papa Francis." So much for no hierarchy.
I get the feeling that Rutherford would be more aptly named "asshole" than "pappy."