In Norse mythology, Jormungandr, also called the "Midgard Serpent", is a gigantic serpent that haunts Midgard until the day of Ragnarök.
He has the god Loki as his father and the giantess Angrboda as his mother. When the Aesir learned of this evil being engendered by such terrible parents, and prophesized the terrible things he would do, they decided to deal with the monster.
Thor threw him into the sea surrounding Midgard, where he will be trapped until Ragnarök, the day of total destruction. Jormungandr grew so big that by biting his tail he could embrace the whole Earth.
The Old Norse etymology of the name is as follows: jörmun (related to Old Saxon "Irmin", Proto-Germanic *ermunaz) meaning "great, exalted, huge, important".
This stem is found in numerous proper names (e.g. Ermengarde, Arminius) gandr or ganðr (related to English "wand", flexible staff or rod, and to "to wind", to twist or bend, Proto-Germanic *gandaz or *wandaz) is a poetic masculine noun describing a magically powerful object or a being imbued with that power.
The object normally associated with gandr is thus a staff. It is the name sometimes used to describe Odin's staff or spear; the root forms the first part of proper names known as Gandalv, a name Tolkien would borrow for his character Gandalf. Accordingly, Jormungandr literally means "the great staff," in the sense of a very large and powerful magical object.
Jormungandr's arch enemy was the god Thor. Three legends stand out in which their clashes are depicted.
In the first encounter, Thor meets the serpent, which has been disguised as an enormous cat by the giant king Utgard Loki. As one of the challenges set by Utgard Loki, Thor was to carry the cat; as he was unable to carry such a monstrous creature as Jormungandr, he managed to lift it high enough to separate one of its four legs from the ground. When Jormungandr is revealed by Utgard Loki, the lifting of the cat is considered one of the most impressive feats.
For a feast organized at Aegir's abode there was not enough mead for all because Aegir did not possess a cauldron large enough to brew it. Aegir set Thor the task of searching for a cauldron, but neither Thor nor anyone else could find one, until Tyr informed him that Hymir the giant, his father, possessed a cauldron a mile deep.
They set out in search of Hymir's palace to borrow the cauldron. On seeing Thor arrive, the giant was enraged, however he sacrificed three oxen for dinner, of which Thor ate two whole, and declared that the next day they would eat fish.
The next morning Hymir and Thor went out fishing, and ventured into seas too deep, to the domain of the Midgard serpent. Once there, Thor cast his fishing rod and Jormungandr caught it, fighting frantically against his opponent. As the god cast hateful glances at the great serpent, it spewed forth seas of venom.
Hymir, seeing the great Midgard serpent, flinched and cut the line before the ship was wrecked, so that Jormungandr could escape. By returning to the giant's palace and giving further proof of his strength, Thor was able to take the great cauldron with him.
It is said that Jormungandr fed on what he found in the sea of Midgard but reached a point where the world serpent was not satisfied and out of desperation it ate itself, causing it to satisfy its hunger by destroying itself but growing more, thus continuing until Ragnarök and thus forming the symbol of the serpent eating its tail, the infinite cycle of destruction and creation of the nine worlds (inspiration of the ouroboros).
It is believed that the world serpent by eating itself for so long obtained the power to see its birth and death, with this it has the knowledge of the future and past. And what is his destiny, that makes Jormungandr one of the wisest and consuming himself in the sea of Midgard quietly awaiting the Ragnarök.
His last encounter will occur with the coming of Ragnarök, when Jormungandr will crawl out of the ocean and poison the skies. From its jaws will swarm the poison and crawl through the fire at the feet of the giants to the place where Thor is to fight against his nemesis and, he who had tried so many times to hunt him, will kill it and then walk nine steps before falling dead from the poison of the great serpent.