excitement gained from an often simple action or occurence; often used to explain reason in response to a stupid question.

to do something just for the hell of it.

To do something for amusement or to annoy someone else.

just for fun, a little kick in the pants

the fun of it, the sake of doing it, thrills, sensation, "funsies", the hell of it, because it's there, "the devil made me do it"

To do something for leisure. To do something for one's own self pleasure.

means "just for fun"