Toponymy
Celtic place-name in *-ākon “place of”, then, “place belonging to, territory of” > Old Welsh -auc, -awg > Welsh -og. The form -iog with an additional /i/ can be explained by the preceding element, which is a personal name ending with -i-us : Festinius. The whole name should be *Festiniākon. Probably same name as Festigny (France, e.g. : Festigny, Marne, Festiniacus in 853)
Read more about this topic: Ffestiniog