Where does “シニア” come from?

シニア (Japanese) comes from English senior, from Latin senior, from Latin senex, from Proto-Indo-European sénos — old.

シニア (Japanese): being senior relatively older

Definitions

Step	Language	Word	Meaning
1	English	senior	Older; superior; Higher in rank, dignity, or...
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