Son of Eöl whose treachery led to the fall of Gondolin, translated “Sharp Glance” (S/133). His name is a combination of maeg “sharp” and glîn(n) “gleam, glint” (SA/maeg, glîn; WJ/337).
Conceptual Development: In the earliest Lost Tales, his name was G. Meglin (LT2/164), and remained N. Meglin in Silmarillion drafts from the 1930s (SM/136, LR/274). The earliest iteration of this name might be tied to G. meg “any small animal, (esp.) mole” (GL/57), since this character was the leader of the house of the Mole (LT2/175); hat-tip to Ivar the Boneless for this suggestion. In Silmarillion revisions from the 1950s-60s, Tolkien considered several other replacements: Glindûr, Targlîn, Morlîn and Morleg (WJ/91, 323), but ultimately settled on Maeglin (WJ/122, note §119).
maeg (“sharp, piercing, penetrating”) + glîn (“gleam, glint [of eyes]”)