Flat Is Justice – a comedic term born in the anime and ecchi community, which then spread to the hentai community to a lesser degree.

The term means “flat chested” and is indeed referring to women’s breast sizes. Specifically, those who are flat as a board or close enough.

While the term seems offensive, flat is justice is in fact a positive connotation and an appreciation of flat chested women (the opposite of what happens in real life you could say).