One of the earliest cases of the expression occurred in the Japanese manga, titled Shuffle! which was released in 2004.
In the visual novel, one of the characters, Mayumi Thyme is giving voice to her pride concerning her flat chest, stating the it is a status symbol.
This was referenced by an episode of the anime series Lucky Star in 2007, which further popularized the notion of flat superiority among fans.
In 2011, the phrase “flat is justice” was heard in Steins;Gate as Mayuri Shiina attempts to hearten her friend Luka Urushibara.
Following 2011, “flat is justice” would appear in online discussions from sites like GameFAQs, to Facebook groups as well as Tumblr, Reddit and 4chan.