aller à la selle
The French idiom “aller à la selle” means literally “to go to the saddle.” But it doesn’t really have anything to do with horse riding. What it literally means is “to go to the toilet,” or “to have a bowel movement.”
The saying is somewhat dated, so it is not used very frequently these days, but you will still come across it in books, magazines and journals.
“La selle” means “the saddle.” It is a word that dates back to the 13th century. It comes from the Latin word “sella,” which meant a seat. The idea of a seat related to the knight who carried out his duties in a seated position, because he was sitting on a horse. So the word came to be used for saddles.
The forerunner of the modern toilet was a chair with a hole in it. This was called “une selle aisée” (a comfortable seat), “une selle nécessaire” (a necessary seat), and “une selle percée” (a seat with a hole in the middle).
The expression itself dates to the 15th century.
Equivalent English expressions include “to go to the can” and “to do a number two.”