It's officially called French toast.
Reason being that in medeival times they were really poor so had to use whatever food stocks were left.
Usually there would be eggs and stale bread (french especially because it goes off so quickly) left so thesewould be beaten together and fried to make a meal.
It did'nt actually originate in France it was just that their bread went out so quickly and was the widest used for the purpose in those days