It means "Green Fuck Nigga!" This refers to making a shot "green" or making a shot "wet" ( when the ball doesn't hit the rim as it goes in). The shooter can say Green FN as they make the shot.
Cody: "You not finna make this shot."
Daniel: "Green FN!!!"
“Green fn” refers to a nba clip of someone making a bucket and saying “green fuck nigga”. “Green” because the bar of the rim turns green on a successful bucket. “Green fn” is now synonymous with spitting smooth shit or game.
It means green fuck nigga, and describes something cool or suave usually in basketball. It comes from NBA2K where the color green measures up to be a perfect shot.
Influencer: If I make this shot, your first @ owes you an ass pic
*makes the shot*
commenter: green fn