Etymology
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nipple (n.)

1530s, nyppell, "protuberance of a mammalian breast," in a female the extremity where the milk-ducts discharge, alteration of neble (1520s), probably diminutive of neb "bill, beak, snout" (see neb), hence, literally "a small projection." Used from 1713 of any thing or mechanical part that projects like a nipple. From 1875 in reference to the mouthpiece of an infant's nursing-bottle. Earlier words were pap (n.2), teat. A 16c.-17c. slang term for a woman's nipples was cherrilets.

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papilla (n.)

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.

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papilloma (n.)

"a tumor resembling a nipple," 1866, a modern Latin hybrid from papilla "nipple" + -oma "tumor," which is from Greek.

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papillary (adj.)

"of, pertaining to, or resembling a nipple," 1660s, from Latin papilla "nipple" (see papilla) + -ary.

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dug (n.)

"animal nipple," or, contemptuously, "the human female breast," originally with reference to suckling, 1520s, origin obscure, perhaps related to Swedish dagga, Danish dægge "to suckle."

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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.

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epithelium (n.)

1748, Modern Latin (Frederick Ruysch), from Greek epi "upon" (see epi-) + thēlē "teat, nipple" (from suffixed form of PIE root *dhe(i)- "to suck"). Related: Epithelial.

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pacifier (n.)

1530s, "one who pacifies or appeases," agent noun from pacify. The meaning "nipple-shaped device for babies" is recorded by 1904. Pacificator "a peacemaker" (1530s) is directly from Latin.

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mastoid (adj.)

"breast-shaped, teat-like, resembling a (female) breast or nipple," 1732, from Greek mastoeides "resembling a breast," from mastos "(woman's) breast" (see masto-) + -oeides "like," from eidos "form, shape" (see -oid). As a noun, 1800, from the adjective.

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tit (n.1)
"breast," Old English titt "teat, nipple, breast" (a variant of teat). But the modern slang tits (plural), attested from 1928, seems to be a recent reinvention, used without awareness of the original form, from teat or from dialectal and nursery diminutive variant titties (pl.).
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