I used to hear this when I was a kid (back during the Renaissance) when someone choked on a bit of food. "No," (cough, cough) "I'm okay now. It just went down my Sunday throat."

Many people saved their best clothes for special occasions, such as going to church on Sunday, so—something you usually use only on Sunday. But this time (choking) you inadvertently used it when you shouldn't have.

Nothing too complicated about it.

Don Firth