A debunked theory that predates the ability to study DNA.

https://en.wikipedia.org/wiki/Khazar_hypothesis_of_Ashkenazi_ancestry

The Khazar hypothesis of Ashkenazi ancestry, often called the Khazar myth by its critics,[1][2] is a largely abandoned historical hypothesis that postulated that Ashkenazi Jews were primarily, or to a large extent, descended from Khazars

In 2013 Martin B. Richards stated that presently available genetic studies shows that 50-80 percent of Ashkenazi Y chromosome DNA could be traced to the Near East, while his own study at the University of Huddersfield found that 80 percent of Ashkenazi mitochondrial DNA could be traced to Europe, but with virtually no lineages from the North Caucasus. This implied a trend of European women marrying Near Eastern men, but provided no evidence to support the Khazar hypothesis.[85]