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.