Green FN

it basically means "Green Fuck Nigga!"
This originates from 2K when a green shot occurs when you nail the timing and release so that it has a high chance of going in to the basket. Green shots are considered the goal for many who play the game, a "Green FN"

It is usually used when players about to make a shot and scream it , they make it, but most likey terribly missed it, or they are trying to showcase a nice move into the basket and misses it.
It became widespread from a picture with peter griffin shooting a basketball in a monocrome background and the caption says Green FN

Oh my goodness Matthew Koscal passes to ball to William Semler for the half-court three. Semler yells "Green FN Confidently!!" HE BANKS THE SHOT!!!!!!