Jormungandr (World Serpent) (Old Norse Miðgarðsormr; also Jörmungandr) is in Germanic mythology a sea serpent that encircles the world (Midgard) and lives in the primordial ocean.
Like the Underworld Goddess Hel and the wolf Fenrir, he was also fathered by Loki with the giantess Angrboda and thus belongs to the arch enemies of the Aesir gods. Thor encounters him three times and twice he fails to slay him.
The meaning of the name Jormungandr is not handed down in the Edda poems and by the skalds. There it is spoken of Jörmungandr ("earth wand" - a kenning for "mighty monster"), Ormr or Naðr (snake, dragon). The lexeme "Jörmunr" (the mighty one) is also used in the Thulur as a kenning for Odin.
In the "Song of Hymir" (Hymiskviða), which belongs to the Liederedda, the myth of "Thor's fishing expedition" is described, in which he rows out together with the reluctant giant Hymir to catch fish, because his unbridled appetite has emptied Hymir's larder prematurely.
Before doing so, Thor rips the head off one of Hymir's bulls to tie it to the line of his fishing rod as bait. Finally, the Midgard serpent bites, and Thor manages to pull it out of the water, where he tries to slay it with a blow from his magic hammer Mjölnir.
Hymir, however, upon seeing the Midgard Serpent, cuts the line, allowing the beast to escape. Thor hurls his hammer after it, but the outcome of the confrontation remains open. According to the Edda, Thor and Hymir both return from the fishing trip; in the Prose Edda, Thor slays Hymir out of rage and returns to land alone.
During his stay in Utgard, Thor meets the Midgard Serpent, which the giant king Utgardloki has used magic to disguise as a giant cat, a second time. In a contest, Thor is given, among other things, the task of lifting the enormous cat to prove his strength. Thor does not succeed in lifting the monstrous creature completely, but he is able to lift it so far that it leaves the ground with one leg. When Utgardloki later dissolves the mirage, he is impressed by Thor's strength.