Not all Aussie terms are slang. ‘Above-ground pool’, for instance, is regular nonslang language unique to Australia. So too is the word ‘ offsider’ (in the sense of an assistant, friend or mate), another expression coined here. Most overseas dictionaries describe an offsider as a player in the wrong place on a football field.The Australian meaning arose from a bullock-driver’s assistant being called an offsider. He was so called because he walked on the off side of the bullock team, while the bullocky himself walked on the on side beside the team’s leader and cracked the whip. From this, offsider was extended to anyone who was an assistant in any occupation or enterprise.The earliest citation for this distinctively Australian use of offsider is from 1879. It’s nice to know that when you refer to your mate as your offsider you’re recalling the role the bullockies played in building Australia.