The term comes from the NBA 2K series. In those games, the quality of a basketballshot is measured with a colored meter. Green signifies the most effective shot, but it is also the most difficult to hit. The FN means "fuck n--ga"
"Green FN" is basically the video game equivalent of shouting "Kobe!" while shooting a crumpled-up piece of paper into the trash.
Trending phrase on Social Media. A reference to making a excellent release shot ‘Green’ in Basketball video Game ‘NBA 2K’
‘Dot me, Dot me!!… ‘Green fn’ the corner sitter shouted after lacing a wide open 3 in the corner (the corner sitter been waiting for the pass since 1763)