Good question. I didn’t know either. Here’s the etymology:

“A side note: To remember the word rez-de-chaussée, a bit of etymology might be useful. Une chaussée is another word for “road,” and rez is Old French for ras, meaning “flat” or “level” (think of the word “razor”). The ground floor is called le rez-de-chaussée in French because it is level with the road.”

Le rez-de-chaussée – Yabla French – Free French Lessonshttps://french.yabla.com › lesson-Le-rez-de-chaussée-258

Fascinating. It must be where our word raze comes from.