Where does “seior” come from?

seior (Vulgar Latin) comes from Latin senior, from Latin senex, from Proto-Indo-European sénos — old.

Ancestry of “seior”, step by step

Step	Language	Word	Meaning
1	Latin	senior	older, elder; rather old; a lord, sir; an abbot
2	Latin	senex	old, aged, elderly; old person, old man, old...
3	Proto-Indo-European	sénos	old

seior — Etymology, Origin & Word History | Etymology Explorer