The word is likely to be derived from the Afrikaans term "plaasjapie" (Japie: first name) translated meaning "Japie of the farm (plaas)". No jarpie in Afrikaans or SA English.
Jacques Basson Cape Town
In Australian/Kiwi/South African backpacking culture, it is commonly believed that this was onomatopoeic from the use of "Yaar" (sounding harsh even to Australian ears ;), to mean "Yes", especially while listening. ie those that say "Yaar, Yaar" as a (mildly annoying?) habit while listening are referred to as 'Yarpies'. Quite likely this is apocryphal but might be a reason contributing to the uptake of the word.