The well defined answer is French Toast originated as an exclusive way to use day-old or stale bread. French breads are commonly known as becoming stale after a day.
Whereas a stale, crunchy bread may seem unappetizing, soaking the bread in eggs and frying it solved the problem for many. The precise origins of the recipe are unknown, although a version appears in the 4th Century CE Roman cookbook attributed to Apicius.
There you have it French Toast from French Bread....YUMMMMMY!!