Hmmm...I came upon this thread by chance, but having an interest in traditional Celtic culture, I now feel that I want to understand the distinction, if one exists. Are there English synonyms for "friend" which more closely approach the meaning of "ffrind" and "cyfaill"? Or, rather, is it the case that they are synonymous, and that "ffrind", having entered the Welsh language, simply supplanted "cyfaill" in spoken Welsh for only that particular nominal sense while leaving the terms derived from "cyfaill" in place? Would using "cyfaill" for "friend" in speech now be seen as incorrect, or rather as correct but just a bit antiquated? Tegs, I see you are in Ireland. Do you know of any Gaelic cognates to "cyfaill"? I don't know how much cognation there is between a Brythonic and a Goidelic language in this late age. Thanks for your time (I know I have a lot of questions, but this stuff fascinates me).
Perhaps "ffrind" came to be used for "friend" in spoken Welsh because of the double meaning of "cyfaill" (either "friend" or "fellow")?