-ις

• -ῐ̈ς (-ĭ̈s), -ῐ̈́ς (-ĭ̈́s) — after a vowel if no diphthong is formed
• -τῐ́ς (-tĭ́s) — extended form

Uncertain, but compare Latin -idus (forming adjectives). Furnée and Beekes also consider it a common Pre-Greek suffix, related to -ιτ- (-it-), -ινδ- (-ind-) and -ινθ- (-inth-).

(This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium.)

-ῐς or -ῐ́ς • (-ĭs or -ĭ́s) f (genitive -ῐδος or -ῐ́δος); third declension

1. Forms feminine nouns, or feminine forms of adjectives (often accented on the ultima)

   ‎Ἕλλην (Héllēn, “Greek man”) + ‎-ις (-is) → ‎Ἑλληνίς (Hellēnís, “Greek woman”)

   ‎Ἰλλυριός (Illuriós, “Illyrian man”) + ‎-ις (-is) → ‎Ἰλλυρίς (Illurís, “Illyrian woman”)

   ‎Τιτάν (Titán, “Titan”) + ‎-ις (-is) → ‎Τιτανίς (Titanís, “Titanid”)

2. Suffix added to a father's name to form a feminine patronymic: daughter of

   ‎Περσεύς (Perseús, “Perseus”) + ‎-ις (-is) → ‎Περσηΐς (Persēḯs), literally “sprung of Perseus”

   ‎Πέρσης (Pérsēs, “Perses”) + ‎-ις (-is) → ‎Περσηΐς (Persēḯs), literally “sprung of Perses”

   ‎Νηρεύς (Nēreús, “Nereus”) + ‎-ις (-is) → ‎Νηρηΐς (Nērēḯs, “Nereid”), literally “sprung of Nereus”

• → Latin: -is

• -σῐς (-sĭs)

• Smyth, Herbert Weir (1920), “Part III: Formation of Words”, in A Greek grammar for colleges, Cambridge: American Book Company, § 843.b.2
• Furnée, Edzard Johan (1972), Die wichtigsten konsonantischen Erscheinungen des Vorgriechischen (Janua linguarum. Series practica; 150) (in German), The Hague and Paris: Mouton, page 324
• Beekes, Robert S. P. (2014), “70 -ῑδ-”, in Stefan Norbruis, editor, Pre-Greek: Phonology, Morphology, Lexicon, Leiden: Brill, →ISBN, § 2.3 The Suffixes, page 36