The story
Calque from Japanese 貧乳(ひんにゅう)は正義(せいぎ) (hinnyū wa seigi), from a fansub translation of a scene from the tenth episode of the anime series Steins;Gate.
Etymology from Wiktionary, CC BY-SA 4.0.
flat is justice
Today it means: Used to express a preference for flat-chested fictional characters, especially in a lolicon context. Full definition →