Fun is usually associated with hobbies and pastimes where the person undertaking the activity enjoys themselves - such as golf. This does not usually include work... unless you are anally retentive or an accountant.
Time spent 'having fun' usually passes in the blink of an eye and in no way balances out the misery of commuting, working, or dealing with the bloody council. However it's all we've got, so best have as much fun as you can, in any way you can, because it's something that some cnut will try and take away from you.
Fun is a state of mind not understood by Muslims in general. This is because they are miserable sods at best and, quite frankly, wouldn't know fun if it jumped out of the Koran dressed in three postage stamps and covered in Dream Topping. See the IFA for details of Islam's idea of fun.
Fun can often involve alcohol at some stage. Unfortunately too much alcohol can lead to a negative fun quotient i.e. misery.