Where does “meini” come from?

meini (Welsh) comes from Welsh -i, from Old Welsh -im, from Proto-Brythonic *-iβ̃.

Ancestry of “meini”, step by step

Step	Language	Word	Meaning
1	Welsh	-i	Pluralisation suffix; Forms verbnouns from verb...
2	Old Welsh	-im	—
3	Proto-Brythonic	*-iβ̃	—