pap (n.1)

"soft food for infants, gruel, porridge," late 14c., from Old French pape "watered gruel" and Medieval Latin papo, both from Latin pappa, a widespread word in children's language for "food" (compare Middle High German and Dutch pap, German Pappe, Spanish, Portuguese papa, Italian pappa), imitative of an infant's noise when hungry; possibly associated with pap (n.2). Meaning "over-simplified idea" first recorded 1540s.

"nipple of a woman's breast," late 12c., pappe, first attested in Northern and Midlands writing, probably from a Scandinavian source (not recorded in Old Norse, but compare dialectal Swedish pappe), from PIE imitative root *pap- "to swell" (source also of Latin papilla "nipple," which might rather be the source of the English word, papula "a swelling, pimple;" Lithuanian papas "nipple"). Like pap (n.1) supposed to be ultimately of infantile origin.

"father," also "older man," 1844, American English shortening of papa.