Question: What is the meaning of the name Moriancumer?
Answer: The meaning of the name Moriancumer is not known. However, scholars Reynolds and Sjodahl suggest “the first part of the name, ‘Morian,’ may be related to the Aramaic, ‘Mara,’ which means lord or master. Or to ‘Moreh,’ a teacher. As for ‘cume,’ these scholars suggest that it means “to stand up” or “rise.” The “r expresses the idea of being, or making.”
Reynolds and Sjodahl conclude that Moriancumer means “one who stands up for God.”
“And it came to pass that the brother of Jared did cry unto the Lord...” (Ether 1:35)
Some years after the publication of the Book of Mormon, the Prophet Joseph Smith was residing in Kirtland, Ohio, when Reynolds Cahoon had a son. One day, when Joseph was passing his door, Reynolds called him in and asked him if he would bless and name the baby. The Prophet Joseph did so and gave the boy the name of Mahonri Moriancumer.
When the Prophet Joseph finished the blessing, he told Reynolds that the "name I have given your son is the name of the brother of Jared; the Lord has just shown [or revealed] it to me." That was the first time the name of the brother of Jared was known in the church (George Reynolds, "The Jaredites," Juvenile Instructor, 1 May 1892, 282).
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 202.