In Norse mythology, Jörmungandr, mostly known as Jormungand, or Jörmungand (Old Norse: Jǫrmungandr), or Midgard Serpent (Old Norse: Midgarðsormr), or World Serpent, is a sea serpent, and the middle child of the giantess Angrboða and the god Loki. It was prophesied that Thor would slay it during Ragnarok, but he would then die from the dying beast's spewing venom. Without any explanation, this card's name in the TCG is in fact a mistranslation.
When in Defense Position, "Jormungandr" is shown to have his tail in his mouth. According to Norse mythology, Jormunganr is wrapped around the Earth with his tail in his mouth, and when he lets go, the world will end. This could mean that when "Jormungandr" is changed to Attack Position, he will have "unwrapped" himself from the world by removing his tail from his mouth, and "let go", causing "the world to end" by having its controller take 3000 damage (a potentially devastating amount by any account).
The ancient Egyptian symbol of the Ouroboros is also depicted as a snake with its tail in its mouth, though it is not holding the world together.
This card shares the same name spelling with "Yormungarde".
This is the only "Nordic" monster that was not shown in card form in the anime.