Where does “dan y don” come from?

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

Ancestry of “dan y don”, step by step

dan y don traces back through 3 separate lines of ancestry.

via Welsh y

Step	Language	Word	Meaning
1	Welsh	y	the; that; which, whom
2	Middle Welsh	y	his, its with reference to a masculine noun; triggers soft mutation of following consonant
3	Old Welsh	ir	the
4	Proto-Celtic	sindos	this
5	Proto-Indo-European	sḗm	one
6	Proto-Indo-European	sem-	together, one

via Welsh tôn

Step	Language	Word	Meaning
1	Welsh	tôn	tone, sound; tune, melody
2	Middle Welsh	tonn	—
3	Proto-Brythonic	*tonn	—
4	Proto-Celtic	tundā	wave, billow
5	Proto-Indo-European	(s)tewd-	to push, to hit
6	Proto-Indo-European	stew-	to praise, laud

via Welsh dân

Step	Language	Word	Meaning
1	Welsh	dân	third-person plural future of dod; Soft mutation...

Every word from Proto-Indo-European sem- →