"Thjálfi was swiftest-footed of all men;"
|Thor vs. the sleeping Skrymir|
Inside the thumb.
Thor and his companions traveled with this Giant through Jotunheim, during which time Skrymir prodded and provoked Thor (under the guise of amiability), and Thor tried and failed to kill him while the rest of his party pretty much shook in their boots. Lucky for everyone, Skrymir didn't seem interested in more than testing them, and left them all in Utgard, where Thor and his companions (Thjalfi included) prevailed upon the hospitality of Utgarda-Loki (not to be confused with Loki the Trickster, though Utgarda-Loki is very much deserving of the same title, as you will soon see!).
Utgarda-Loki scorned Thor and his companions pretty openly. And why shouldn't he when the god had shown up on his doorstep, in enemy territory, dependent upon his goodwill for food, drink, and a bed? Utgarda-Loki demanded that Thor and his companions prove their worthiness and their skill before he allowed them to stay in his hall, and of course Thor and his companions rose to the challenge (all except Roskva, who appears to have been given a free pass, either because she was a woman, or because she was just a girl, it isn't all that clear).
First, Loki claimed he could eat faster than any man in Utgarda-Loki's hall. He was beaten by a man named Logi who ate not only the meat, but also the bones along with it, and the trough in which the food was laid out.
Second, Thjalfi claimed he could outrun anyone that Utgarda-Loki chose for him to race against. Thjalfi ran three heats against a boy named Hugi, but though Utgarda-Loki said he had never seen anyone as swift as Thjalfi against Hugi, the poor Thjalfi still lost all three attempts, and by a wide margin.
Finally, it was Thor's turn. He claimed he could outdrink anyone in the hall (sheer quantity of liquid, not liquor-holding, though I suspect he would have been a champion in that department as well). Utgarda-Loki handed him a large drinking horn, filled to the brim. According to Snorri:
Then said Útgarda-Loki: 'It is held that this horn is well drained if it is drunk off in one drink, but some drink it off in two; but no one is so poor a man at drinking that it fails to drain off in three.'Thor tried. Really hard. But he couldn't down the contents of the horn in one, two, or three drinks. In fact, he had barely lowered the level of the liquid. At this point, someone had to have realized that SOMETHING fishy was going on in this hall, but Thor was too busy getting worked up about his own failure, and I'm sure Loki was busy being sick from speed-eating while Thjalfi was walking off his failed races.
Utgarda-Loki gave him a second chance. Kind of. Framing the challenge as insultingly as possible, Utgarda-Loki challenged Thor to pick up his cat off the ground. Thor, puffed and no doubt furious at this point, immediately took up the challenge. But try as he might, he could only lift the cat enough to move ONE of its paws from the floor. Now. I imagine by now even Thor was starting to figure out that the game was rigged, but he wasn't going to give up of course, because Thor can always, always be counted upon to take up a challenge and gosh darnit, he was going to prove himself to Utgarda-Loki if it KILLED him to do it!
So. Utgard Loki offered him one last
Utgarda-Loki, having thoroughly shamed and humiliated Thor and his companions, then saw fit to give them a place to sleep and a good breakfast, all friendliness and hospitality. He even came out to see them on their way the next morning, and it was only then that he told Thor that NOT ONLY had it been Utgarda-Loki himself who was the giant Skrymir, but also this:
'Now I will tell thee the truth, now that thou art come out of the castle; and if I live and am able to prevail, then thou shalt never again come into it. [...]So it was also with the games, in which ye did contend against my henchmen: that was the first, which Loki did; he was very hungry and ate zealously, but he who was called Logi was "wild-fire," and he burned the trough no less swiftly than the meat. But when Thjálfi ran the race with him called Hugi, that was my "thought," and it was not to be expected of Thjálfi that he should match swiftness with it.And that drinking horn? Thor was drinking the SEA itself. The cat was none other than the Midgard Serpent in disguise. And the old lady he wrestled. Oh, she wasn't any old lady. She was Old Age, personified.
As far as Thjalfi's story goes, I would say it is no small thing to be so fast that the only thing that can outrun you is thought. Not bad for a peasant's son!
And that is the Story of Thjalfi, Bondservant to Thor, according to the Prose Edda by Snorri Sturluson.