Polari 

Used mainly by gay men in the UK during the 20th century, it is mostly a lexicon, derived from many different languages. These include: rhyming slang, back slang, cant,  Italian, French, Yiddish, and other types of slang. While it is debated on the classification of the lexicon and if it truly is a sociolect, an anti language, a slang vocabulary, or a language. Each persons knowledge of the language was different, they would've known different terms, pronunciations, and spellings depending on where they learned it from. It was not often written down, therefore it is very difficult to research and class this vocabulary. One of the most prolific users of the lexicon was the Julien and Sandy show which was a 1960s radio show that brought Polari into the twinkling starlight; but this was also its Achilles heel, now that the broader population knew what words meant, it was no longer safe to use it as a protective code.  Source 

Lets break it down; 

Nanti is a negation used to mean none, no, nothing, don't, and beware. 

Naff  is something that is awful, tasteless, boring, lame, or straight

Therefore, Nanti Naff-> Not boring or beware the tasteless.