The second branch of the Mabinogi, the story of Branwen Uerch Lyr, is well-known among students and connoisseurs of Celtic literatures. Not the least attractive feature of it is that it still hides so many unsolved riddles. It is attempted here to bring one of these nearer to an acceptable solution by looking at it from a wider perspective. The alleged saying in the title of this contribution has never been found attested elsewhere in Celtic literature, except by citation, of course. If it was a fixed expression, or even a proverb when the story became fixed as we have it, it must have had a long history already. The following investigation is an attempt to detect this background by means of historical linguistics and a comparison of IE literary formulas, adducing also archaeological findings.