I had a quick search on the net and the best definition I've found suggests that the word naff comes from a Polari word - naph meaning cheap, nasty or bad. The Polari language is believed to be a mix of English, Italian, Jewish, Irish Gaelic, Romany and anything else that happened to be around which was spoken by travelling entertainers over the past few centuries. It seems to be only a select few Polari words have ever made it into the english language (Khazi for toilet is one !!!)

Ian