A dirty nigger.
Plefegi (plural)
Fegus (singular plefegus but smaller)

You can shout this word anywhere and nobody can stop you, it's not an inside joke, but nobody knows what the true meaning is. To call one a "plefegus" is the most offensive yet best insult you can throw, a conversation-killer if you will.

P.S - This word should never really be used to offend, but more to refer to, further, it can block any swear.