Ar stêr Forth zo ur stêr eus reter Bro-Skos hag a aber er Firth of Forth.
Bodotria e vefe an anv kozh, hervez :
Dielfennadur, hervez Rivet & Smith, p. 269-271 : "It seems that Forth may well derive from *Voritia (*vo- 'somewhat' plus a root as in Irish rith 'act of running', that is 'slow running one', this *Voritia being later name or by-name ...", da lavaret eo ur stêr difonn he red.
Hervez A.D Mills : "The Celtic river name probably means 'the slow flowing one", "an hini a red difonn".