NAFF, I believe is from a secret language, called "Polari", used by Gay people in the olden days. i.e. 50s/60s.
It meant Not available for f***ing. But was also used as a general derogatory term and became popular in England in the 70s as a mild expletive. Princess Anne famously told some reporters to "Naff off" when they were bothering her.
