The suffix ‑erior is used in many words that seem to indicate position:
However, with my Google-fu, I can’t find a real definition or etymology.
What does ‑erior mean? From what language does it come?
Your Google-fu probably fails because the suffix is "-ior", not -"erior". For example, there are the words "excelsior", "senior", "junior".
The Concise Oxford Dictionary of English Etymology says:
So, it comes from Latin via French.