Historically, surnames were little encountered in Europe until the middle ages. Only then did the practice of adopting a family name become prevalent. Jews began to use surnames much later, towards the end of the 18th century.
A German act of 1787 required all Jews to register a permanent family surname of Germanic origin by 1808. Often families adopted names to suit religious occupations. Kahn, and its many variations, for example, means ‘priest.’ Other European nations introduced similar laws.
Kahn, along with many other names, is a derivation of Cohen, a name coming from Kohein, the Hebrew word for priest. Jews from Germany are known as Ashkenazic.
For more information on customs relating to Jewish names, the following website is recommended: