This is a reference to a specific myth about Thor. From the Wikipedia-entry on Jörmungandr:

In one story, Thor encounters the giant king Útgarða-Loki and has to perform deeds for him, one of which is a challenge of Thor’s strength. Útgarða-Loki goads Thor into attempting to lift the World Serpent, disguised by magic as a huge cat. Thor grabs the cat around its midsection but manages to raise the cat only high enough for one of its paws to leave the floor. Útgarða-Loki later explains his deception and that Thor’s lifting the cat was an impressive deed, as he had stretched the serpent so that it had almost reached the sky. Many watching became fearful when they saw one paw lift off the ground. If Thor had managed to lift the cat completely from the ground, he would have altered the boundaries of the universe.