I was in Malaysia once and a got chatting to a guy who didn't have the best English. He asked me for another way to say "I'm only joking" and I taught him to say "I'm only having a giraffe", which he managed in broken English ("Having a jee raff") I hope one day he says that to an Englishman and confuses him thoroughly.
Fuckwit is a traditional insult that I have found is often quickly taken into new arrivals hearts. My guess is, this is because it unites two words with very old IndoEuropean roots.