Aussie formal?
Some RAAF mates were guests at a barbi' in the UK a couple of years ago. Noting the chilly weather and overcast sky, one chap turned to his hostess and announced "We're not used to having to wear sweaters at a barbi' - in Oz it's usually just singlets and thongs!"
There was a sharp intake of breath from the hostess - until it was explained to her that 'thongs' are also the Oz term for flip-flops!