It's because the Prince of Wales, Britain's regent ruler during the 1810s and the reason the Regency period got its name, was ... well, let's just say he got pretty portly over the years. When his stylishly-tight clothing got to be uncomfortable, he'd unbutton something. Finally, he just started leaving the bottom button open all the time.
So when your guy gets dressed up in a three-piece suit, he's paying homage to a British ruler from two hundred years ago.
The illustration here is of George as a young man -- or perhaps the artist just knew how to flatter his patron.
Here's another interesting tidbit. In the US we call the third piece of a three-piece suit a vest, but in Britain, it's called a waistcoat. What Brits call a vest is what we American's call an undershirt. So you can imagine how silly it sounds to a British reader when an American author refers to leaving the lowest button of a vest unfastened. Sigh.
As playwright George Bernard Shaw once said, England and the United States are two countries "separated by the same language." And that brings up a question. Have you faced a situation where it was tough to make yourself understood -- or tough to get what the other person was talking about -- because of an oddity in the language?
