Fortrenn is derived ultimately from the tribal name Verturiones

The tribal name contains ver very and *treno strong. A link can be made using the second element to the Turones a tribe on the Loire who have left their name in Tours.

The surviving form "Fortrenn" reveals the copyist's error, the likely Britonnic form being *vertrenos. An identical inversion of "r" and "u" occurs in the form "*turones".