Hahaha what a wrong guess I threw out there.

Yeah that’s the bogan use. It’s derived from ‘septic tank’, which rhymes with ‘yank’ so shortened, it’s seppo. It comes from WW2 I understand when they were over here in droves and pissed the locals off.

So yeah if you ever see an Aussie say ‘fuck off seppo’ you know the background and the depth of the insult.