Normally made whenever someone makes a crazy shot or trick shot and a image pops up with Peter Griffin doing a jump shot in basketball. Since commonly in fortnite it means green fortnite
After he made a trick shot and landed a headshot he said "Green fn"
Green fn stands for green fuck nigga that's used to refer to someone who is particularly suave or cool. In its original use case, the term was fairly specific to the world of basketball. That's because the term originated from NBA 2K.