The famous saying created by Mario Fenech when they would play him getting hit in the head by a mis-directed pass on the Footy Show each week. Basically it refers to an unusual happening or sight that the speaker is perplexed by. Since it was used so much it became a greeting of sorts where it was a lead in to a joke by the recipient. It gained international appeal as a greeting compliments of Contiki in 1998 across Europe for travellers from the States, Canada, Asia, Africa and South America.
When you see a bloke cheering on Manly at Shark Park you say "What's Doing?"