Where does “seu” come from?

seu (Portuguese) comes from Portuguese Senhor, from Latin senior, from Latin senex, from Proto-Indo-European sénos — old.

seu (Portuguese): Second-person singular possessive pronoun. your;...

Definitions

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