Oppai is the Japanese term for breasts, but it is also utilized in slang talk.
The word was introduced to English speakers in the 80’s and 90’s, when Japanese anime and manga comics started to spread.
It usually refers to unrealisticly large, perfect breasts minimalizing the stimulus inflicted to the virgin brain.
Oppai had spread widely, largely thanks to the T-shirts, featuring the word with a pair of minimalistic breasts, both men and women wear, as well as the rising popularity of Japanese anime and manga series in the western world.
When heard in an English conversation, it is mostly used in relation to a character from anime, manga or a videogame, with the exception of next level otakus referring to their classmates’ breasts as oppai.
In everyday Japanese oppai is more a child’s word for the breasts of their mother.
Oppai had its entry written on Urban Dictionary in 2004 already, with other definitions expanding readers’ knowledge about this aspect of the otaku world.