Fun you means Fuck you in a nice way so you can cuss without being yelled at by your parents!
Apple 1:Im gonna get eaten hha in ur face apple 2!
Apple 2:thats not very nice!!
Apple 1:what are you gonna do cuss at me ? ahahahha
Apple 2: Yah! Fun you! ahhaha
Apple 1:AHHHHH the worst insult!!!