Stands for 'Not A Fan'
When pronounced as a single word (naf) it is has the secondary yet almost synonymous meaning of something being 'lame' or generally not very good.
This acronym coincidentally spells 'Fan' backwards, adding further depth to it's meaning.
Because of these two notable coincidences, the word is entering common usage throughout the globe.
Person 1: 'Have you seen 'Avatar' yet?'
Person 2: 'Yes, but to be be honest I'm N.A.F'
Person 1: 'I'm N.A.F either, to be honest I thought it was naf'
Person 2: 'Agreed, in fact I'm the reverse of a fan, a naf, if you will'
Person 1: 'Oh, I will'