a language spoken by some Jewish people, which is related to German
(from Cambridge Advanced Learner's Dictionary)

A Jewishlanguage based on German once spoken throughout Central and Eastern Europeby AshkenazicJews. The name comes from a combination of the German words, "Jud" (Jew) and "Deutsch" (German). Sometimes referred to as a dialect of German, and sometimes denigrated as the "Ebonics of the Austro-Hungarian Empire", Yiddish does have its own unique qualities which differentiate it from standard German. Unfortunately, most Yiddish speakers were killed in the Holocaust, and emigration of the Jews of Central Europe and Eastern Europe resulted in the near-disappearance of the language. Today it can only be heard in isolated pockets in certain big cities worldwide. It is probably the most endangered European language right now. However, Yiddish words have heavily influenced American English,especially that of New York, and to a lesser extent have made it into Cockney. The use of Yiddish words like schmuck, putz, shyster, etc. by comedians, in particular, has spread Yiddishisms throughout the Anglosphere. Even though the Yiddish language is in a severe and probably irreversible state of decline, certain Yiddish words will always be with us, kein ahora.

Largely European language of the Jewish population. Based on an amalgum of German syntax and grammar, with vocabulary from German and Hebrew, transcribed in the Hebrew alphabet, with loan words from many other Central and Eastern European languages (Czech, Magyar, Russian, Slovak, Polish).