British slang, today meaning uncool, tacky, unfashionable, worthless... or as a softer expletive, in places where one might use "fuck" as in "naff off", "naff all", "naffing about".
Origins of the word are disputed, but it appears to have come from Polari
(gay slang), used to dismissively refer to heterosexual people. It was introduced as a less offensive expletive verb ("naff off") in the '70s UK television show, Porridge. "Naff off!" was famously used by Princess Anne in 1982.
Naff the naffing naffers.