plural papillae, 1690s, "a nipple of a mammary gland," from Latin papilla "nipple," diminutive of papula "swelling" (see pap (n.2)). Meaning "nipple-like protuberance" attested from 1713.
"small, often inflamed, swelling of the skin," late 14c. (early 14c. as a surname), of unknown origin; perhaps related to Old English pipligende "having shingles;" also compare Latin papula, papilla (see pap (n.2)). As a verb, "to cover with pimples," from c. 1600. Related: Pimples.
"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.