When someone is going to do something really stupid, useless, or annoying, or if you are jealous that someone is getting to do something you're not, it is a put-down. See whatever.
geek: Well, I'M going to get my hair follicle tested for carbon today!
normal kid: Psh, have fun.
Annie: guess what, i get a free shopping spree at neiman marcus today!!
Jenna: psh, have fun.
a common response for whenever someone announces that they are going to do something totally normal, boring, or routine.
Person 1: I'm going to the bathroom.
Person 2: Have fun!
Person 1: I'm going to pick up some milk at the grocery store.
Person 2: Have fun.
A sarcastic response to someone's unfortunate upcoming situation.
Corey: Hey man, where you headed?
Mike: Fucking physics class...
Corey: Have fun! :D