Ascûdgamln, pronounced ASK-KULD-GAM-il-LIN (fists of steel in Dwarvish), were weapons made by the dwarves. They were metal spikes attached to the knuckles of the bearer's hand, enabling the bearer to punch through material he would otherwise be unable to.
How they are made
According to Shrrgnien, who had Ascûdgamln, they were made by putting the would-be bearer into a 'deep sleep', drilling holes through their finger joints, embedding metal sockets into the holes and sealing them in place with magic. Then, various-sized spikes could be threaded through the sockets. Afterward, the dwarf could remove and replace them by screwing them out. Few dwarves attempt this because one can easily lose the usage of his hands. He also explained to Eragon that humans probably could not have this treatment because it had never been done to a human before and their hand bones were thinner, so they might drill right through them.
Eragon wanted to have fists of steel in Brisingr because he had a battle where he and Arya fought a group of soldiers with their bare hands. Eragon didn't try to get them because he was afraid his hands would be destroyed in the process. He later used a spell to create thick calluses on his hands to stop him from hurting his hands when he punched with the strength of an elf.
The Ascûdgamln are reminiscent, to a degree, of Brass knuckles. Also, the character Wolverine in the "X-men" comic series has retractable blades protruding from his knuckles, somewhat resembling Ascûdgamln, though the Ascûdgamln aren't retractable.