Yid, is the Yiddish word for Jew. It has its origins in the Medieval High German word Jüde (the contemporary German word is Jude.)
Compare to the word Yiddish, parallel to standard high German jüdisch, meaning Jewish. Historically, the word Yiddish is an adjective derived from the original noun Yid.
When used by non-Jews the word yid (but not Yiddish) is usually meant to be offensive. However, Yiddish speaking Jews proudly call themselves Yidden (plural for Yid).
