Hev
It's Kaffir.
And it is EXTREMELY offensive.
Always.
To everybody in South Africa - black or white.
I know - from a first hand experience with a Dutch South African. My attempt at jocularity with him is something I will always regret, it lowered me below the level of the gutter in his eyes. And, if the person you are with does not find it a contemptible term, you should ask yourself whether they deserve your company.
All - please take heed and don't use it - it's as bad / worse(?) than the n- word in the USA.
stales
Edit: Seems I got it right. Found the following quote at an informative (pointed) essay about South Africa at http://www.uwlax.edu/ereserves/cox/soc225/south_africa.htm "In Arabic, kaffir originally meant infidel, today it is a racial slur like the n word in US"