From The Collaborative International Dictionary of English v.0.48:
Yiddish \Yid"dish\, n. [G. j["u]disch, prop., Jewish, fr. Jude
Jew. See Jew, Jewish.]
A language used by German and other Jews, being a Middle
German dialect developed under Hebrew and Slavic influence.
It is written in Hebrew characters.
[Webster 1913 Suppl.]