It means breasts or boobs, most often used in a non formal way, for example when consulting a doctor at a check up in japan they would not use the word oppai; however, a young boy going through puberty in Japan might use oppai when talking about his crush’s breast.
Hope that helped you understand not only the meaning but when and where it may not be appropriate to use the word oppai
“Oppai” means boob in Japanese. A formal Japanese word for chest is “Mune”.
“Oppai” could also be used to mean “breast milk” in some context.
I think these words sound similar to oppai.
“Shoppai”, that means “salty”.
ex) クラブでおどったあとのあのこのおっぱいは、ちょっとしょっぱかった。”Her boob after dancing at the club was little salty.”
“Suppai”, that means “sour”.
ex) クラブでおどったあとのあのこのおっぱいは、ちょっとすっぱかった。”Her boob after dancing at the club was little sour.”
Ippai”, that means “a lot”
ex) かれのあたまはおっぱいのことでいっぱいです。”He’s obsessed with boobs”