Where does “defnyddwyr” come from?

defnyddwyr (Welsh) comes from Welsh defnyddio, from Welsh defnydd, from Celtic *damniyom, from Proto-Indo-European dem- — to build; to arrange, to put together.

Ancestry of “defnyddwyr”, step by step

defnyddwyr traces back through two separate lines of ancestry.

via Welsh defnyddio

Step	Language	Word	Meaning
1	Welsh	defnyddio	to make use of, employ, practise; to form,...
2	Welsh	defnydd	use, service; application; purpose, end, effect;...
3	Celtic	*damniyom	—
4	Proto-Indo-European	dem-	to build; to arrange, to put together

via Welsh ŵyr

Step	Language	Word	Meaning
1	Welsh	ŵyr	Soft mutation of gŵyr; grandson, grandchild
2	Middle Welsh	wyr	Soft mutation of gwyr

Every word from Proto-Indo-European dem- →Every word from Celtic *damniyom →Every word from Welsh defnydd →