Stephen D. Ricks et al. discusses the etymology of "Mormon"; argues it may derive from mr mn "truly beloved."
Dictionary of Proper Names and Foreign Words in the Book of Mormon, ed. Stephen D. Ricks, Paul Y. Hoskisson, Robert F. Smith, and John Gee (Orem, Utah: The Interpreter Foundation, 2022), 232, 235-36
Brief Etymology: The Lehite PN MORMON is based on Egyptian roots meaning “truly beloved” or “strong/firm love” or “love remains steadfast/firm.”
Extended Discussion: Robert F. Smith and Benjamin Urrutia have each called attention to the Egyptian words mr mn, “truly beloved,” or “love is established,” or strong/firm love” or “love remains steadfast/firm” (RFS). The translation “love is established” being to mind the words of Paul, “charity never faileth” (1 Corinthians 13:8). Interestingly, it is MORMON who uses the same words in a letter written to his son Moroni, adding, “But charity is the pure love of Christ, and it endureth forever” (Moroni 7:46-47) (JAT).
As Carl Hugh Jones pointed out in 1970, Frederick G. Williams left two possible hieratic characters from the Book of Mormon plates which he claimed represent the phrase “Book of Mormon,” and Egyptologist John Gee has said that it looks authentic, because the character for “book” in that phrase is written in a fashion nearly identical to a known early Demotic Egyptian form of md3t, “book.” That the syntax appears to be Mrmn m md3t (reading from right to left) is no problem since names or titles frequently come first in Egyptian, as in md3t ntr, “Holy Writ,” where ntr is written first.
On a limestone stele of the Nineteenth to Twenty-First Egypitan dynasty in the Museum of Gizeh, the name mrmnu appears, accompanied by the title “door keeper.” In a yet-to-be-superseded article, Wilhelm Spiegelberg treats the name mrmnu as Semitic in Egyptian transcription although he is not certain that it is Semitic, and he does not provide a meaning. He transcribes it into Hebrew characters with mr/lmn(w). Spiegelberg’s description of the stele does not permit its current identification. Despite various difficulties, such as dating to at least six hundred years before Lehi and not having an etymology, this name, mrmn, on an Egyptian inscription seems like a direct hit, as Hugh Nibley pointed out years ago. Unfortunately, no immediate etymology suggests itself.
Nevertheless, Nibley has also suggested that mrm is attested in Hebrew and Arabic, and means “desirable” or “good.” In this case, MORMON would consist of the root √mrm plus the Semitic/Hebrew ending -ān = -ôn/-ōn (often used in Hebrew GNs and PNs, such as Kidron and Gideon). For possible Hebrew examples, see the biblical PN mirmāh in 1 Chronicles 8:10, and the PN merēmôt (also of questionable value for an etymology; Septuagint μεραμωθ), the name of a priest in Ezra 10;36 (=Nehemiah 10:5), re possibilities (JCH). The name also appears as mrmwt on a sixth-century BC ostracon from Arad. Note also the PN at Ugarit, ma-ri-ma-na (JCH), but the language origin of the name is unknown.
The Egyptian mr (> Nubian and Coptic mur, mor), “bind, girth,” is somewhat less likely (RFS).