When said quickly, an inoffensive way to advise someone to 'Fuck Off'. Best used discretely so that only people close to you can hear it, and not the intended victim. Even funnier when the intended victim THINKS they can hear it, but can't be sure, so they take no action. However, if victim is being a complete tosser, best to shout it as loud as possible in order to deliver as much offence as possible. Number of 'a' letters contained in word is directly proportional to how much you want the individual to fuck off.
'Excuse me, is anyone sitting on this chair?', 'No, you can have it - (Faaff!)'
'Sorry Sir, you can't come in here wearing jeans', 'FAAAAAAAAAAAAAAAAAAAAAAFF!'