I have heard several theories of how the name "Trelfa" came about. One idea is that it is a corruption of the french for metal cutter. Another is that it orriginated from the "Threlfall" name (an aristocratic family in Lancastershire). The theory I grew up with (and my favourite despite the lack of proof) is that it is Cornish: the rhyme "by Tre, Pol and Pen, you shall know the Cornish men" is the only evidence.