rabbi (n.)

"Jewish doctor of religious law," early 14c. (in late Old English in biblical context only, as a form of address); in Middle English as a title prefixed to personal names, also "a spiritual master" generally; from Late Latin rabbi, from Greek rhabbi, from Mishnaic Hebrew rabbi "my master."

This is formed from -i, first person singular pronominal suffix, + rabh "master, great one," title of respect for Jewish doctors of law. This is from the Semitic root r-b-b "to be great or numerous" (compare robh "multitude;" Aramaic rabh "great; chief, master, teacher;" Arabic rabba "was great," rabb "master").

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Definitions of rabbi from WordNet
rabbi (n.)
spiritual leader of a Jewish congregation; qualified to expound and apply Jewish law;
Rabbi (n.)
a Hebrew title of respect for a Jewish scholar or teacher;