Literally, "best friends just friends". Used to describe a guy and a girl who attend events, like a party, together but as strictly friends. Similar to a mutually consensual friend zone
Origin: Tessa's apartment in Phoenix, 2012. From the Latin, "Justus Friendsus".
Jack and Jill roll up to a party. Megan, who likes jack, says "Hey Jack, who's the girl?"
Jack says, "Don't worry, we're just BFJF."