Where does “senior” come from?

Senior comes from Latin senior, comparative form of senex meaning "old," derived from Proto-Indo-European sénos.

senior (English): Older; superior; Higher in rank, dignity, or...

Definitions

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