Kosali is considered as an ancient language by researchers and it is one of the five Prakrit languages existing since Vedic era along with Sanskrit. But the Kosali speaking region is affluent with adivasis. And they must be using a language to communicate before Sanskrit era. It seems, after the invasion of the Aryan, the adivasis of the region may have adopted the language of the mainstream. Even many other caste and tribe have been migrated to this region in the course of time. All have brought their own culture and language and amalgamated it into the language of the present Kosali speaking region.
Further, the Kosali speaking region was ruled by Nala, Naga, Sarabhapuriya, Chola, Ganga, Somavansi, Kalachuri, Chouhan, Maratha, and Britishers. Even after the acquisition by Britishers, this region like a football has been once in Bengal, once in Madhya Paradesh (Central Province) again with Bengal and then Bihar and finally settled with Odisha. And in the course of time the present form of Kosali may have evolved with an influence of all these transition. The degree of influence might be varying for each factor; insignificant to significant.
At present Kosali is an Indo-Aryan language. It is been categorized under the languages of Purvi Hindi Language Group. The languages of this group are, Awadhi, Baghelkhandi, Chattisgarhi and Kosali.