Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

Yiddish
     n : a dialect of High German including some Hebrew and other
         words; spoken in Europe as a vernacular by many Jews;
         written in the Hebrew script