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.
2. Can be used in virtually any context that "fuck" can (but not used to denote coitus) only way classier, and sounds much better.
2. naff off, I can't be naffed, stop naffing around, etcetera