the rather old-fashioned but terribly polite "I have to go and powder my nose".
I remember distinctly the first time I
noticed that (though I had almost certainly heard it before). A friend and I, sophisticated men of the world at 17, decided to take our girlfriends to a rather smart restaurant. At the end of the meal, as the time to pay approached, one of the girls said to the other, "Shall we go and powder our noses?"
I remember being struck by the quaintness of the euphemism. My grandmother and mother powdered their noses frequently - a shiny nose must have been something no 'lady' would ever expose, but teenage girls in the early 1960s never powdered their noses. They were either fresh-faced and (apparently) free of make-up, or covered in a thick mask that lasted for days without needing a top-up.
The second thing that struck me, some time later, was the wordly wisdom of the girl who suggested the powdering. Sophisticated people (and that was what we were trying to be) never allowed a situation in which the ladies would see the gentlemen perform the sordid task of handing over money to the waiter. The departure of the ladies to powder their noses was the signal for the waiter to bring the men the bill.
As we are well off-topic, I'll go even further astray. Until I protested violently at the farce about six years ago, my wife, influenced by her own upbringing, would, if she was taking me out for a meal, always discreetly hand me the money under the table (if she had forgotten to give it to me beforehand) when the time came to pay. She did not go as far as going to powder her nose, but she refused to publicly pay for both of us. She was a professional woman, more highly qualified than I, and earning more money - indeed, at the time of my protest, she was my boss.
In the business world she would openly pay her share when dining with colleagues, but to be seen to be paying for the meal of a male was too much for her.