British slang word with varied uses, not all certainly connected; see Partridge, who lists three noun uses: 1. "female pudenda" (c.1845), which might be back-slang from fan, shortening of fanny (in the British sense); 2. "nothing," in prostitutes' slang from c.1940; 3. a euphemism for fuck (v.) in oaths, imprecations, expletives (e.g. naff off), 1959, "making it slightly less obvious than eff" [Partridge]; and an adjective naff "vulgar, common, despicable," said to have been used in 1960s British gay slang for "unlovely" and thence adopted into the slangs of the theater and the armed forces.
Not very good; lacking taste or style (1966+)
(also about)To goof off, fool around
[1959+; British euphemism for fuck off]