The term Down Under is a colloquialism which is variously construed to refer to Australia, New Zealand, and other countries in Australasia. The term comes from the fact that nearly all of these countries are in the Southern Hemisphere, 'below' many other countries on the globe.[1][2]
But something tells me that you have your own opinion on where the term "Down Under" came from. Something involving discrimination and arrogance by those in the Northern Hemisphere.