I wonder whether this is where 'frock' comes from.
Either that, or all their dresses were amphibious. :Þ
The other way around; ffrog was borrowed from English frock according to http://geiriadur.ac.uk/gpc/gpc.html?ffrog .
All the links to Geiriadur Prifysgol Cymru that I've clicked on lead me to a pag with only a logo, no text at all; what can I do to access the text? Diolch.