From the Dutch phrase "Voort seg ik!" literally meaning "Forward say I!". It was then adapted by South Africans into voetsek, and then eventually "footsack" in English.
It was originally meant for dogs, but is now fairly commonly used against people as well, although in this context it's meant as a light-hearted "fuck off!"
The most common variation is the abbreviated "tsack", or the colloquial "tsek".
Dopie: Can I have a bite of your hotdog?
Salie: Footsack!
(or, more realistically)
Dopie: Gimme a stuk of your boerewors roll!
Salie: 'tsek!
Used in a tweet by @zigged on 21-Aug-2009: "Following @TastyBakingCo because I'm happy to find them on Twitter. TastyKakes were the foodtrack of my childhood."
Funyuns are the featured artist in the foodtrack of my youth.
Chocolate is the base/bass of my life's foodtrack.