Ratbag ...

[naz_nomad]

Any idea where this term originated, and why it's considered an insult?

Thanks .. Naz x

[Quizmonster]

It originated as Australian/New Zealand slang in the 1930s to mean a troublesome, uncouth or stupid person.

[naz_nomad]

so ... nothing to do with rats then?

[Quizmonster]

Well, describing someone as a rat has been a form of abuse since the 1500s. Shakespeare himself used the word thus.
Presumably - given the way Aussies often tend to go over the top with their phraseology - they imagined a bagful of rats was that much more nasty than just the one! Perhaps rat-catchers on cattle stations finished a day's work carrying a literal 'ratbag'...not a pleasant thought!