This article discusses the development of the contrastive-partitive function of the possessive =eš in colloquial Persian. Examples of colloquial Persian show that the third person singular clitic pronoun =eš in some adnominal possessive constructions does not refer to any obvious referent present either in the syntactic structure (co-text) or in the situational context. Instead, the function of =eš, namely contrastive-partitive, is to mark the host as a part and contrast it with other parts of the similar set. The same function is attested in a few languages of Uralic and Turkic group. We believe that the same development has been occurred in possessive =eš in Persian. To describe the process of the development of the contrastive-partitive function, authentic colloquial examples from Internet blogs and formal examples from a historical corpus of New Persian are investigated. It is argued that this non-possessive function of =eš has originated from the whole-part relation in cross-referencing possessives, where both the lexical and clitical possessor =eš are present. The presence of the lexical possessor facilitates the loss of referentiality in =eš and it is developed to denote partitivity. Furthermore, the pragmatic motivation of communicating contrast makes =eš to be further grammaticalized into denoting contrastive-partitive function.