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Just for fun means you're too lazy to do anything else and you're just messing around because you're bored and you don't care about anything.
Bro just opened a pizza box and started eating it like it was a life-or-death situation. J4f.
I spent 2 hours arguing with my dog over who gets the last chicken nugget. J4f.
I decided to write this definition at 3 a. m. because I was too tired to sleep. J4f.