Gwyddyl Ffichti is a term that appears in the third series of Welsh Triads, apparently referring to the Picts.
It was central to William Forbes Skene's argument that the Picts were a Goidelic, Celtic-speaking people and that their language was ancestral to modern Scottish Gaelic.
[1] The passage in which it appears is believed to be an invention of the 18th/19th century Welsh Antiquarian Iolo Morganwg.
[2] The suspicion of Morganwg's forgery was first raised by Skene himself in 1868: It is a peculiarity attaching to almost all of the documents which have emanated from the chair of Glamorgan, in other words, from Iolo Morganwg, that they are not to be found in any of the Welsh MSS.
[4] Skene revised his position on the nature of the Pictish language to suggest it was an amalgamation of "Welsh" and "Gaelic": It has been too much narrowed by the assumption that, if it is shewn to be a Celtic dialect, it must of necessity be absolutely identic in all its features either with Welsh or with Gaelic.