[m; pl. “Rabboteinu”]; literally, our “Master,” or “Teacher of Torah.” The idea of “Mastery” in connection with the teaching of Torah is firmly in place, although Judaism abhors the concept of slavery of Man to Man. The sense in which the expression is used here is basically that a dedicated student, or congregant, is obligated in general to follow the decisions in Jewish Law of his or her Teacher or Rabbi, which is based on the same root, and has a similar meaning.
The Chassidic Communities tend to extend the idea of “mastery” beyond the area of Jewish Law, and to consult and follow the decisions of their “Rebbe,” in areas outside the range of Torah Law.
The most famous example of a “Rabbeinu” is, of course, “Moshe Rabbeinu,” “Moses, our Teacher.”