Grand Rebbe

Rebbe (Hebrew: רבי‎‎) /ˈrɛbɛ/ or /ˈrɛbi/, is a Yiddish word derived from the Hebrew word rabbi, which means "master, teacher, or mentor". Like the title "rabbi" it refers to teachers of Torah or leaders of Judaism.

In common parlance of modern times, the term "The Rebbe" is often used specifically by Hasidim to refer to the leader of their Hasidic movement.

The Yiddish term rebbe comes from the Hebrew word rabbi, meaning "My Master", which is the way a student would address a master of Torah. It was an honorific originally given to those who had Smicha in the Pharisaic and Talmudic era. Since vowels were not written at the time, it is impossible to know historically whether it was pronounced rah-bee (/ˈrɑːbi/) or r-bee (/ˈrɛbi/). The English word rabbi (/ˈræbaɪ/) comes directly from this form. In Yiddish, the word became reb-eh (/ˈrɛbɛ/)—now commonly spelled rebbe (/ˈrɛbə/—or just reb (/ˈrɛb/). The word master רב rav [ˈʁäv] literally means "great one".

...
Wikipedia