Afrikaans slang which loosely translates to the derogatory insult "fuck all you fuckers," or "fuck you all." Made popular by South African rap-rave group DIE ANTWOORD, but not invented by.
Naai is a South African Afrikaans slang term meaning fuck.
It originates from the literal Afrikaans word of the same name which means 'stitch' or 'sew'.
It is most commonly spoken in the Western Province city, Cape Town and, as with the word fuck, it's meaning's vary depending on the use of the word.
It can be used in reference to sex or having sex as well as an expression of disagreement or distaste. It's also quite commonly used as an impolite way of referring to someone.
In reference to sex:
Joe: You know, I hate getting leg cramps when I naai.
Tony: Me too, but your mom doesn't seem to mind.