The root form of Kazvin is; 'kaz oyni,' meaning 'Kaz's playground'
Mahmud Kashgari: The root form of Kazvin is; 'kaz oyni,' meaning 'Kaz's playground'
محمود كاشغرى: ريشه كلمه قزوين٬ قاز اوينى به معنى بازيگاه قاز٬ دختر خاقان ترك افراسياب است
7 AACAR BULLETIN (ASSOCIATION FOR THE ADVANCEMENT OF CENTRAL ASIAN RESEARCH)
"مشاهدات محمود كاشغرى (نويسنده كتاب ديوان الغات الترك٫ قرن 10 ميلادى) در باره افراسياب محتاج نگاهى نزديكتر مىباشد. در توضيح در باره دختر "كوز" اطلاعات جالبى داده مىشود: "كوز " نام دختر افراسياب ٫ كسى كه شهر "قزوين" را ساخت مىباشد. شكل ريشه اى اين كلمه "كاز اوينى" به معنى محل بازى كاز به تركى مىباشد زيرا كه وى در آنجا ساكن بود و بازى مىنمود...."
The observations of Mahmud Kashgari require a closer look. Explaining the word "Kent" (city), he wrote: "Among the Oghuz and those who associate with them, it means town among the Turks. The chief city of Ferghana is called Oz Kent, meaning 'city of our souls.' Samiz Kent meaning 'fat city,' is called thus because of its great size; it is, in Persian, Samarkand." [DLT, P.173]
In explaining Afrasiyab's daughter Koz, interesting information is provided: "Name of the daughter of Afrasiyab. She is the one who built the city of Kazvin. The root form of this is; 'kaz oyni,' meaning 'Kaz's playground' since she used to live there and play. For this reason some of the Turks reckon Kazvin within the borders of the Turk lands.
Also the city of Qum, since: Qum is in Turkic 'sand' and this daughter of Afrasiyab used to hunt there and frequent it.
Others of them reckon (the borders) from Marv as-Shahijan since her father: Tonga Alp Er--who is Afrasiyab--built the city of Marv, three hundred years after Tamhurat built the citadel. Some of them reckon all of Transoxiana as part of the Turk lands, and in the first place: Yarkand (Baykand). This used to be called Dizruin, meaning (in Persian) 'city or castle of brass' because of its strength. It is near the city of Bukhara." [DLT, P.509]