-ίτης

Ancient Greek

Back-formation from a rebracketing of πολῑ́της (polī́tēs, πόλῐς (pólĭs, “city”) +‎ -της (-tēs, demonym suffix)), by reanalysing it as πόλ- (pól-) +‎ -ῑ́της (-ī́tēs).

-ῑ́της • (-ī́tēs) m (genitive -ῑ́του, feminine -ῖτῐς); first declension

Suffix forming a masculine noun: one connected to, a member of; one from a particular place (demonym)

Originally formed generic adjectives, such as ὁπλῑ́της (hoplī́tēs, “hoplite”, from ὅπλον (hóplon, “large shield”), literally “one with armour”); πολῑ́της (polī́tēs, “citizen”, from πόλῐς (pólĭs, “city”), literally “one from the city”) etc.
By the Hellenistic period, both masculine -ῑ́της (-ī́tēs) and feminine -ῖτῐς (-îtĭs) became very productive in forming technical terms for products, diseases, minerals and gems (adjectives with elliptic λῐ́θος (lĭ́thos, “stone”)), ethnic designations and Biblical tribal names. These technical uses survive in modern languages in Greek loanwords for diseases (in -itis), for minerals (in -ite) and for nations, tribes or religious sects (plurals in -ites).

Case / #	Singular	Dual	Plural
Nominative	-ῑ́της -ī́tēs	-ῑ́τᾱ -ī́tā	-ῖται -îtai
Genitive	-ῑ́του -ī́tou	-ῑ́ταιν -ī́tain	-ῑτῶν -ītôn
Dative	-ῑ́τῃ -ī́tēi	-ῑ́ταιν -ī́tain	-ῑ́ταις -ī́tais
Accusative	-ῑ́την -ī́tēn	-ῑ́τᾱ -ī́tā	-ῑ́τᾱς -ī́tās
Vocative	-ῖτᾰ -îtă	-ῑ́τᾱ -ī́tā	-ῖται -îtai
Notes:	This table gives Attic inflectional endings. For declension in other dialects, see Appendix:Ancient Greek dialectal declension.

Smyth, Herbert Weir (1920), “Part III: Formation of Words”, in A Greek grammar for colleges, Cambridge: American Book Company, § 843