おっぱい

Generally agreed upon as originating from baby talk.[1][2][3][4]

Derivation unclear. Possibilities include:

• A shift from ををうまい (wo wo umai, literally “huh huh delicious”), as something a baby might say while breastfeeding.

/wo wo umai/ → /o o umai/ → /oːmai/ → /opːai/

While suggestive, the phonetic changes required seem unlikely. Moreover, the pitch accent differs, in that うまい (umai) has a high pitch on the ma followed by a downstep, while おっぱい (oppai) has a high pitch on the initial o followed by a downstep.

• A shift from 一杯 (ippai, “one serving, one cupful; full”),[4] as something a baby might say when done breastfeeding, or when asking to do so.

/ipːai/ → /opːai/

This seems more likely, as the phonetics align better: only one vowel changes, and the pitch accent in ippai is similar with a high pitch on the initial i followed by a downstep.

• (Tokyo) お​っぱい [óꜜppàì] (Atamadaka – [1])[3][5][4]
• IPA^(key): [o̞p̚pa̠i]

おっぱい or オッパイ • (oppai) 

1. (childish) milk, breastmilk (liquid)

   Synonyms: 乳 (chi, chichi), (adult word for breastmilk) 母乳 (bonyū)

   おっぱいが出(で)るようになる

   oppai ga deru yō ni naru

   to become able to produce milk

2. (anatomy, childish) boobies, titties (breasts)

   Synonyms: パイオツ (paiotsu), 乳 (chi, chichi), 乳袋 (chibukuro), 乳房 (chibusa), (more general meaning of chest) 胸 (mune)

   • 2000 August 10, Yūma Andō with Asaki, Masashi, “[喧](けん)[嘩](か)[屋](や)　[国](くに)[光](みつ)”, in サイコメトラー[EIJI](エイジ), volume 24 (fiction), Tokyo: Kodansha, →ISBN:

     キャサリーン！オッパイ　大(おお)きくなれよぉー！

     Kyasarīn! Oppai ōkiku nare yōo!

     Katharine! Your boobs! Grow them bigger, okay!?

   • 2015, 谷川ニコ [Nico Tanigawa], “喪79”, in 私がモテないのはどう考えてもお前らが悪い！, ガンガンONLINE [Gangan Online]:

     八(や)ツ橋(はし)のやわらかさは小(しょう)学(がく)生(せい)のおっぱいと同(おな)じ…　まじかよ　京(きょう)都(と)人(じん)ってど変(へん)態(たい)だな…

     Yatsuhashi no yawarakasa wa shōgakusei no oppai to onaji… maji ka yo Kyōto-jin tte do-hentai da na…

     Yatsuhashi are as soft as the tits of an elementary schooler… You serious? Kyoto must be seriously perverted…

• (childish) ぱいぱい (paipai)
• (breasts): (reversal slang) ぱいおつ (paiotsu)
• (breasts): (slang) パイスラッシュ (paisurasshu)
• (slang) 雄(お)っぱい (oppai)

• Chinese: 歐派／欧派 (ōupài)

• Entry at Gogen Allguide (in Japanese)