-ensis

Unknown. Various theories have been put forward:

• Hermann Gähwiler (1962), building on a suggestion by Manu Leumann, proposes a borrowing from Etruscan on the basis that its earliest attested non-toponymic use is in the term ātriēnsis, and the Roman ātrium was an Etruscan import.^[1] A similar suffix is attested in Etruscan patronymics.^[2]
• Paavo Castrén (1981) rejects the relevance of ātriēnsis, viewing it as a later development. On the basis of various ancient, then-extinct peoples of Latium with names in -ēnsēs mentioned by Pliny the Elder, Castrén situates the emergence of the suffix in the late Proto-Villanovan period, perhaps as a borrowing from another Italic language.^[3]
• Chantal Kircher-Durand (1983) suggests a derivation from Proto-Indo-European *-went-ti (< *-wénts, *-tis),^[4] but considers Gähwiler’s thesis plausible.^[2]
• A. Zimmermann (1921) suggests a lost cognate of Ancient Greek ἐνς (ens), variant of εἰς (eis) (< Proto-Indo-European *h₁én) + -ī.^[2]

• (Classical Latin) IPA^(key): /ˈen.sis/, [ˈẽːs̠ɪs̠]
• (modern Italianate Ecclesiastical) IPA^(key): /ˈen.sis/, [ˈɛnsis]

-ēnsis (neuter -ēnse); third-declension two-termination suffix

1. Of or from [a place].

The suffix -ēnsis is added to a toponym (especially the name of a town) or to a topographical name, in order to form an adjective.

Examples:

‎Eborācum (“York”) + ‎-ēnsis → ‎eborācēnsis (“of or from York”)

‎castra (“camp”) + ‎-ēnsis → ‎castrēnsis (“of the camp”)

Third-declension two-termination adjective.

• -ānus / -īnus