After Fëanor's death, Morgoth captured Maedhros through trickery, and hung him by the wrist from the heights of Thangorodrim. Fingon succeeded in rescuing him, but he lost his right hand in their escape.
After the defeat of Morgoth in the War of Wrath, Maedhros and his brother Maglor stole the two remaining Silmarils from the camp of the Valar, but the hallowed Jewels would not suffer the brothers to hold them. In torment, Maedhros cast himself into a fissure in the earth.
This meaning appears in The Etymologies, but there are some indications that Tolkien may not have meant to retain this derivation. Other possibilities include 'Spray of Light' or (according to some sources) 'Well-formed One', but 'Glint of Metal' seems to be the only derivation attested to by Tolkien himself. (The History of Middle-earth Volume 5, The Lost Road and Other Writings III The Etymologies).