Jormungand, (also spelled Jormungandr), in Norse mythology, the evil serpent that encircled the world, biting its own tail. Jormungand (which means "wolf-serpent") was also known as the Midgard Serpent, or the World Serpent, because its body coiled around the whole Earth underneath the oceans, and the Earth was called Midgard in Norse cosmology. The serpent's eternal enemy was the thunder god Thor. Jormungand was one of three monstrous offspring of the giantess Angerbotha and the trickster fire god Loki. Jormungand's brother was the giant wolf Fenrir, whom the gods had to leash, and its sister was the goddess Hel, queen of the underworld. Jormungand's venom was utterly poisonous and its size was immense. So the father of the gods, Odin, cast the serpent into the sea, where ever afterward its movements were responsible for fatal storms and tidal waves.
The 'Prose (or Younger) Edda' recounts how Thor almost slew Jormungand. In the land of giants, Thor went fishing with the fisherman giant Hymir. Hymir, unaware that his companion was the thunder god, refused to recommend any particular bait, so Thor chopped off the head of Hymir's largest ox, named Himinhriot, and put it in the boat. Thor rowed skillfully, taking the boat out into the open sea, far beyond the place where the giant usually fished. He fastened the ox's head to the hook and threw out his line. The Midgard Serpent, languishing in the ocean bed, stretched its mouth around the ox head and was caught fast on the hook. Using his tremendous strength, Thor hauled the serpent up and locked eyes with him. Thor was about to kill the serpent with a blow from his magic hammer, Mjolnir, when Hymir, in a panic that his boat would capsize and terrified at the sight of the serpent, cut the line with his bait knife. The serpent escaped and sank back into the sea. Thor threw his hammer after it, and while some believed the serpent dead, the 'Prose Edda' says, "the Midgard Serpent lives still and lies in the encircling sea."
In another encounter, a magician-giant tested Thor's strength by challenging him to lift a grey cat. The cat was so heavy that Thor only succeeded in lifting one paw off the ground. Although the giant feigned contempt for the puny deed, he later confessed that he was astonished at Thor's feat, for the cat was really the Midgard Serpent, and Thor had lifted it "not far from the sky."
At the end of time, during Ragnarok, the final battle between the forces of good and evil, Thor was destined to encounter Jormungand again. He would at long last kill him with a hammer blow, but he would himself survive only long enough to take nine paces from the serpent before succumbing to its poison. In this way, the Midgard Serpent and Thor would kill each other at the end of the world

the story of Midgard Serpent and Thor

Thor went out of Asgard disguised as a youth and came in the evening to a giant called Hymir. Thor stayed there that night. At daybreak Hymir got up and dressed and prepared to go sea-fishing in a rowboat. Thor sprang up and asked Hymir to let him go rowing with him. Hymir said that he would not be much help, as he was such a scrap of a young fellow: "You'll catch cold if I sit as long and as far out to sea as I usually do."
Thor, however, said he would be able to row a long way out from the shore, and that it wasn't certain that he would be the first to demand to be rowed back. He became so angry with the giant that he was ready to set the hammer ringing on his head. He controlled himself, however, as he intended to try his strength in another place. He asked Hymir what they were to take as bait, but Hymir told him to get his own. Then Thor turned away to where he saw a herd of oxen belonging to Hymir. He selected the biggest ox, one called Sky-Bellower, and struck off its head.
Thor took the ox-head on board, sat down in the stern, and rowed. Hymir thought they made rapid progress from his rowing. Hymir sat in the bow, and together they rowed until they came to the place where Hymir was accustomed to sit and catch flat fish. Thor said he wanted row much farther out, and they had another bout of fast rowing. Then Hymir said that they had come so far out that it would be dangerous to sit there on account of the Midgard Serpent (Midgardsormen).
Thor, however, declared his intention of rowing for a bit yet, and did so, and Hymir was not at all pleased at that. When Thor shipped his oars, he made ready a very strong line and a large hook. He baited the hook with the ox-head and flung it overboard.
The Midgard Serpent snapped at the ox-head, and the hook stuck fast in the roof of its mouth. It jerked away so hard that both Thor's fists knocked against the gunwale. Then Thor grew angry and, exerting all his divine strength, dug in his heels so hard that both legs went through the boat until he was digging his heels in on the sea bottom. He drew the serpent up on board, staring straight at it. The serpent glared back, belching poison. The giant Hymir turned pale with fear when he saw the serpent and the sea trembling in and out of the vessel too. At the very moment that Thor gripped his hammer and raised it aloft, the giant fumbled for his bait knife and cut Thor's line off at the gunwale.
The serpent sank back into the sea. Thor flung his hammer after it and people say that this struck its head off in the waves; but the truth is that the Midgard Serpent is still alive and is lying in the ocean. Thor clenched his fist and gave Hymir a box on the ear so that he fell overboard head first, but Thor himself waded ashore.