"pimple, small inflammatory elevation of the skin," 1864, from Latin papula "pustule, pimple, swelling" (see pap (n.2)). Papula in the same sense is attested in English from 1706. Related: Papular.
Entries linking to papule
pap n.2
"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.