Where does “o hyn allan” come from?

o hyn allan (Welsh) comes from Welsh hyn, from Middle Welsh hynn, from Old Welsh hinn, from Proto-Celtic sindos, from Proto-Indo-European sḗm, from Proto-Indo-European sem- — together, one.

Ancestry of “o hyn allan”, step by step

o hyn allan traces back through 3 separate lines of ancestry.

via Welsh hyn

Step	Language	Word	Meaning
1	Welsh	hyn	in conjunction with the definite article these
2	Middle Welsh	hynn	—
3	Old Welsh	hinn	—
4	Proto-Celtic	sindos	this
5	Proto-Indo-European	sḗm	one
6	Proto-Indo-European	sem-	together, one

via Welsh o

Step	Language	Word	Meaning
1	Welsh	o	he, him; of; from
2	Proto-Brythonic	o	from
3	Proto-Celtic	*au	away from
4	Proto-Indo-European	h₂ew	away from, off; again
5	Proto-Indo-European	ǵʰe	Postpositional intensifying particle: indeed, at any rate, in fact

via Welsh allan

Step	Language	Word	Meaning
1	Welsh	allan	out; outbound; Soft mutation of gallan
2	Middle Welsh	allan	outside, out; forward, forth, on

Every word from Proto-Indo-European sem- →