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?
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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:
-ior formerly also -iour — F. -ieur, † -iour — L. iōrem, nom. -ior, suffix of compar. of adj., as in anterior, exterior, inferior, interior, junior, posterior, senior, ulterior. In warrior the ending has another origin.
So, it comes from Latin via French.