Kak, prounced to rhyme with fuck, is an Afrikaans word meaning shit. It has been adopted into South African English, used in much the same manner as shit or crap, that is, meaning a negative value judgement, or just about anything except literal defecation.
Piet: Howzit Frik, How was your date last night?
Frik: Kak! She just talked about her Ex, and I got so drunk that I puked on her shoes. So how was your night?
Piet: Great! I met this model, a really hot chick, and we got on really well. We snogged and I have her phone number.
Frik: Don't talk kak.
This is not just an Afrikaans word. The American Heritage Dictionary gives kakka- or kaka- as an Indo-European root word for To defecate. This is an old and widespread word in Indo-European languages. Webster notes that this word is present in both Latin and Greek. Though in Greek, "kakos" simply means "bad".
There is an English form of this word, cack, which is pronounced in a more English manner, to rhyme with track. However cack is not very commonly used in English, and usually for more literal meanings, as Webster points out in his wonderfully euphemistic definition of cack: To ease the body by stool.
Siobhan points out that die Kacke is German colloquial for shit/excrements. Also in Dutch (unsuprising, given that Afrikaans is so close to Dutch), kak is dung or shit. The American Heritage Dictionary claims that "poppycock" is a corruption of Dutch pappekak, literally porridge-dung, or squishy shit.
Redalien points out that Cach is Welsh for "shit", and is pronounced like kak.
It is possible that khaki is also a related word - apparently it comes from an Persian word meaning dust-coloured, with khak meaning dust.
StrawberryFrog talks kak.