I was reading ‘Llestri’r Dylluan’, a Welsh translation of Alan Garner’s ‘The Owl Service’, when I came across this: ‘Gwyn, paid â hefru arna i’. I had the devil of a job tracking down ‘hefru’, which I couldn’t find in any of my usual dictionaries, until, guessing from the context that it had to mean something like ‘scold’ or ‘be cross with’, I found ‘rhefru’ in the dictionary and decided ‘hefru’ had to be a version of that. Come on, Welsh authors, play fair! We learners accept that you operate under this strange compulsion to change the initial letter of every other word, but nobody has told us about a mutation rule that changes rh to h. So I take it this is not a mutation as such, just an unofficial variant not recognised by dictionaries? If so, are there other rh- words I need to look out for that are likely to appear without their initial r? Or is there actually an unofficial rh to h mutation in some areas that the grammar books don’t tell you about?
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Yup, that - aka colloquialism, aka language shift - I don’t think I’ve ever heard anyone say ‘rhefru’, whereas ‘hefru’ is fairly common. 
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Also the GPC has a note under rhefraf/rhefru (‘backbite, slander, disparage’) saying ‘see also hefraf/hefrio’, which it translates as ‘traduce, run down, disparage’ amongst other things. So it looks like two words with a very similar sound and range of meanings kind of cross-pollinating each other
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