I'm kind of curious about expressions or idioms or references that people outside of your country or your community might find baffling.
My ex from the US could speak about 6 languages fluently but she really struggled to use Australian expressions appropriately. She tried to use "put the hard word on..." and it would come out as "put the hot potato on..." or "put the hot word on...".
Not so much an expression or idiom, but one of the Aussie guys I worked with at the time was always asking girls if they wanted to come back to his hotel room for dutch ovens. She asked me what it meant and I explained that it meant "farting under the blankets". She couldn't believe that we had a term for it that almost all Australians knew.
Another one was a game called "Soggy Sao" which was largely an urban myth. It is supposed to be a game where the competitors must ejaculate on a large, square savoury cookie called a Sao. Whoever misses or the last person to actually hit the Sao has to eat it. She found it extremely weird that every Australian person she knew (about 20 in our little expat community at the time, I would guess) was familiar with the game and the details of how it was played, even though they claimed never to have actually played it. And that even if she asked an Australian person "what's that wet cookie game that you play in Australia?" they all knew what she was talking about.
