-ιον

• -ῐ́ον (-ĭ́on)

Perhaps neuter substantivisation of -ιος (-ios, “adjective suffix”), from Proto-Indo-European *-yós.

For the development from adjective to diminutive, compare Sanskrit -क (-ka) and Proto-Slavic *-ъka. From the development from adjective to place, compare Latin -ārium. (Can this⁽⁺⁾ etymology be sourced?)

-ῐον • (-ĭon) n (genitive -ῐ́ου); second declension

1. Termination of neuter substantivations of adjectives in -ιος (-ios, “belonging to”, also forms parasynthetic compounds).
2. A noun-forming diminutive suffix.
3. Suffix forming names for places.

• -εῖον (-eîon)