Interestingly, while French was the lingua franca of Europe for several hundred years, it wasn't the origin of the term 'Lingua Franca'.

That term meant the "language of the Franks" and was the Mediterranean trade language in the medieval through Renaissance eras. It was actually a pidgin of Italian, French, Greek and Arabic adopted as being roughly mutually intelligible among Venetians, Byzantines and North Africans.

The reference to the 'Franks' is because the generic word for a western European (in the Byzantine, Greek world) had long been "Frank".