Mablung's curious name relates to an incident of historic importance. After the slaying of Carcharoth, it was Mablung who withdrew Beren's severed hand, and the Silmaril it held, from the belly of the monstrous Wolf. However, the Jewel proved heavier than it appeared, forcing Mablung to drop the hand and the Silmaril to the ground.
From this event Mablung took his name 'with weighted hand' (and also his common title, 'of the Heavy Hand', which is essentially a translation of his name). According to one tradition, he had been given this name prophetically, probably many thousand years before he let Beren's hand fall. According to another (perhaps more plausible) account, he acquired the name only after the incident in the Hunting of the Wolf, and it became so well known that it was universally recorded in histories to the extent of completely replacing his original name, which was ultimately forgotten.