A "rack" was a slang term for a motor-bike for older Australians. So, to go get "on-yer-bike" and go away upon a command you would be "racking off". To rack off is less rude, but alliterative, for another popular expression.
Hello mate, may i come to your party and dance the "Maroubra Stomp?"
No, mate, you may not. Rack Off.