When it comes to Aussie phrases my favorites are:
“Don’t come the raw prawn with me.” Meaning don’t try to deceive me by appearing naive or ignorant.
Describing someone as a “black catter”. As in, you have a fast car their car is faster, you have a big house their house is bigger, you have a black cat their cat is blacker.
Old timey slang is lots of fun.
As I said in the OP, a lot of it is sexual. Consider Tudor England.
And one of course can’t appreciate Shakespeare without recognizing the double entendres he frequently used.
In another thread, there’s a reference to “country matters” and the fact that “nothing” was slang for a woman’s genitalia, giving new understanding to the title Much Ado About Nothing
My ex mother-in-law did the same. She told me it was British. I’ve since learned it is not.
I think in certain contexts - when referring to places or things, not people - it means the same as “classy” would to an American. “Posh restaurant” = “classy restaurant”. Whereas when referring to a person, “classy” denotes their behavior, in a purely positive sense, whilst “posh” denotes their background, usually in a negative way.
There’s a version of that I came across in the UK I like- Elevenerifer. As in, you’ve been to Tenerife, they’ve been to Elevenerife.