Mormegil means "the Black Sword".

Túrin was given this name by the Elves of Nargothrond after his sword Anglachel had been reforged as Gurthang. It was always black but its edges shone with pale fire. So great was Túrin's skill in battle upon the Guarded Plain, that the Elves began calling him Mormegil after his black sword.

Encyclopedia entry originally written by Nienna-of-the-Valar