Gordon writes: 'Ac' seems to have been a Celtic suffix denoting 'place of' (rather like AS 'tun'). The placename's first element is almost always a personal name. ====================================================== There was a Celtic leader in Gaul at the time of Julius Caesar whose name ended in -ac to which the Romans added "us." Can't remember it but it's found in a Gerhard Herm book called "The Celts."