Fun is a generic term applied to pursuits that have little constructive meaning and only serve to satisfy the individual(s) concerned. Quiet at the back there!
Having fun is discouraged as it may lead to overactive pleasure glands. And we all know what that leads to now, don't we?
“Er, the Teletubbies?”
Incorrect! You have been having too much fun watching television, child. Detention!
“Aw, can I not have my tea first?”
No, you insufferable urchin! Write 100 times on the blackboard: "I must not eat food because it can be fun".
A brief list of things that are considered 'fun' by some: