The ship built from the dead’s fingernails and toenails is Naglfar.
Naglfar is the enormous death-ship associated with Ragnarök, the catastrophic final conflict of Norse mythology. According to the Prose Edda, it is made from the untrimmed nails of the dead. When the restraints holding it fail, Naglfar sails toward Vígríðr, the battlefield where the gods and their enemies meet.
The Poetic Edda also mentions the ship during its prophecy of Ragnarök, describing it as arriving with rising waters and carrying hostile forces. Snorri’s account names the jötunn Hrym as its captain, while the poetic tradition connects the ship with Loki and other enemies, so details vary between sources.
The nail imagery explains a funerary warning in the Prose Edda: people should trim the nails of the dead, because untrimmed nails add material to Naglfar and help bring about its completion. Scholars have also debated the name’s etymology, but the “nail-ship” interpretation is the familiar one and is the standard answer to this question.