The Jassic people came to Hungary together with the Cumanians, chased by the Mongol-Tatars. They were admitted by the Hungarian king Béla IV, hoping that they would assist in fighting against a Mongol-Tatar invasion. But shortly after their entry, the relationship worsened dramatically between the Hungarian nobility and the Cumanian-Jassic tribes and they left the country. After the end of the Mongol-Tatar occupation they returned and were settled in the central part of the Hungarian Plain.
Initially, their main occupation was animal husbandry. During the next two centuries they were fully assimilated to the Hungarian population, their language disappeared, but they preserved their Jassic identity and their regional autonomy until 1876. Over a dozen settlements in Central Hungary (e.g. Jászberény, Jászárokszállás, Jászfényszaru) still bear their name. The name of the city of Iași in Romania may also derive from the name of the people.
The only literary record of the Jassic language was found in the 1950s in the Hungarian National Széchényi Library. It is a short 1-page glossary containing 34 words mainly related to products of agriculture (types of grain, cattle, etc.) probably compiled for fiscal or merchant purposes. The glossary was interpreted with the help of Ossetian analogies from the Digor dialect.