18th cent. The accent on the second syllable may be due to French influence. From Latin papa 'id.'
Probably of nursery origin; it is not farfetched to suggest a reduplication of Proto-Indo-European *ph2 "father." See also mamá, padre, madre.
Ancient Greek πάππα (páppa) "father"
Armenian (Alaškert) pab "father" (probably a loanword)
Persian bābā "father," Pahlavi pāpak 'id.'
Etruscan papa (?) "grandfather"