You can link directly to a definition (if there is only one for a given sequence of letters) like this: http://www.geiriadur.ac.uk/gpc/gpc.html?dis
According to http://www.geiriadur.ac.uk/gpc/gpc.html?ffristial , ffristial is even older than dis, with citations a century or so earlier. Though as you say, the derived ffrist itself seems a lot younger: http://www.geiriadur.ac.uk/gpc/gpc.html?ffrist
Ap Geiriadur gives definitions from a number of contemporary dictionaries, none of them showing etymology.
The Geiriadur Prifysgol Cyrmu (GPC) is the dictionary in the link given in an earlier comment - it is not referred to by the Ap Geiriadur. The GPC is the standard historical and etymological dictionary of Welsh. Get to it through the link given, or look for the GPC app for mobile devices. Be aware that many of its historical definitions and usages may be unusual in modern colloquial Welsh.
Oh I should have said 'dis' should be 'a die', sorry -- dictionaries are not always correct, just put in words that people commonly use, there are masses of words I would NEVER use from modern dictionaries, sad how our language is going down the pan, but that is another matter.
Joiedevivre777 is absolutely 100 per cent right. a) dictionaries are not the be all and end all when it comes to correct language and b) just because an error is perpetuated doesn't make it become a non-error. Same with apostrophes in the English language, practise/practice (I've mentioned that before but the excuse is that Duolingo is American - my argument being that if it's being "written" (in every sense of the word, ie the normal sense and the software-writing sense) for the UK market, then it should have English English, not American English) and countless other examples. If you use the 'coming into common usage' argument, then we will all be saying: "pls txt me + btw I'm not (happy emoji) about xyz/whatever. BR (best regards) or KR (kind regards). Language is the one thing that differentiates us from wild animals - let's not trash it.