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".

A casual invitation / threat used by an individual that is not afraid of fucking someone up.