I’m reading Tolkien’s translation of Beowulf and in the commentaries he mentions that the Anglo Saxon word rún is probably derived from Celtic and that Rhin in Welsh means ‘secret, mystery, enchantment’, I looked it up in the dictionary and it says virtue or essence. Is Tolkien thinking of an old meaning of the word or does it still have this meaning?
The Welsh Roots and Branches book lists rhin (with the meanings you give) in the same entry as cyfrin which is indeed ‘secret’ and the related words from that root cyfriniol (mystical)l so there’s obvisouly some link.
“cyf” seems a common prefix but not one I’ve managed to quite get a handle on. It turns gweld (see) into cyfweld (interview) and agos (close) into cyfagos (right next to!) and seems to have a sense of both “like that but more so” and a two-way-ness about it.
So I don’t think the example is wrong, just a bit esoteric!
Re “cyf” (cy, cym): this also is seen in words where in English you find “syn/sym” (Greek σύν for “with/together”), e.g. cyfystyr - synonym, cyfamserol - synchronous, syntax - cystrawen, etc. Treating it as basically meaning “with” or “together” often works for me, consistent with your interpretation @leiafee . Cyd looks like a similar construct, e.g. cydymdeimlad - sympathy, cydweithio - co-operate, gyda - with. Maybe that is linked more to Latin “cum/co-” with similar meaning
@cap , Wikipedia says that run- also occurs in Baltic languages, where it means speech, which confirms Tolkien’s ultimate meaning.