The Lamanite word "Rabbanah" (Alma 18:13), meaning "powerful or great king," is strikingly similar to other Semitic words having essentially the same meaning. For example, the New Testament word "rabboni" clearly refers to one who is a leader (John 20:16). Also the word "rabbi," which is used frequently by Jewish people, designates "one who teaches or leads." [Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, p. 207]
According to Glenn Scott, when Ammon came to report and saw the king's odd look he turned to leave. A servant said to him, "Rabbanah, the king desireth thee to stay" (Alma 18:13). That term meaning "powerful" or "great" was obviously of Hebrew derivation. (Note the similarity to "Rabboni" which Mary called the resurrected Jesus (John 20:16). [Glenn A. Scott, Voices from the Dust, p. 147]
According to Hugh Nibley, the term "Rabbanah" is not Hebrew, it's Aramaic. It means "a great one, a great king, a great person, a great wise man." But it means "a person of utter preeminence" with the nah ending. With the nah, it means "our lord." Rab is great, and Rabannah would be "our great one." Notice that these people were Ishmaelites, which is important. That's why they didn't use the Hebrew term for "great king," which would be Melek. [Hugh W. Nibley, Teachings of the Book of Mormon, Semester 2, p. 378]
Alma 18:14 What wilt thou that I [Ammon] should do for thee, O king [Lamoni]? ([Illustration]): Ammon Before Lamoni [Gary Kapp, Verse Markers, Book of Mormon, Vol. 1, p. 2]