Where does “y” come from?

y (Middle Welsh) comes from Old Welsh ir, from Proto-Celtic sindos, from Proto-Indo-European sḗm, from Proto-Indo-European sem- — together, one.

y (Middle Welsh): his, its with reference to a masculine noun; triggers soft mutation of following consonant

Definitions

his, its with reference to a masculine noun; triggers soft mutation of following consonant

y traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Old Welsh	ir	the
2	Proto-Celtic	sindos	this
3	Proto-Indo-European	sḗm	one
4	Proto-Indo-European	sem-	together, one

Step	Language	Word	Meaning
1	Old Welsh	di	to
2	Proto-Brythonic	*ðɨ	to
3	Proto-Celtic	*dū	to, towards
4	Proto-Indo-European	de	Emphatic or contrastive particle, and, but;...