Loki is referring to the shape of the blade on Coal’s sword. I haven’t mentioned it much in the comic, but Coal’s sword has a single-edged blade, which is very unusual for Viking swords, and primarily found in Norway. They require more strength and a bit more finesse to use. Swords themselves were relatively rare weapons (compared to spears or axes or bows and arrows), because they take more time and skill to make. They were often handed down over generations with repairs and changes made as needed.