The story
In popular belief, so named because the hilts sometimes depict a face, which is commonly believed to be that of King Charles I, executed in 1649; in fact, swords of this type with that face were used by both sides of the conflict even before Charles was killed. An alternative explanation for the name is that many such swords were donated to churches when their wielders died, and used in funerary displays.
Etymology from Wiktionary, CC BY-SA 4.0.