The mighty wolf Fenrir is one of the children of Loki with the giantess Angrboda. The Aesir gods so feared the offspring of such an abominable union that they decided to imprison Fenrir.
Fenrir is so strong that the gods had to ask the dwarves to produce a special chain to hold him. They made a chain as fine as ribbon, but forged from impossible things, and therefore impossible to break.
The only way to secure Fenrir with the chain was to trick him into putting it on himself. The gods suggested that he put on the chain to show off his strength by breaking it.
Sensibly suspicious, Fenrir only agreed to participate in the game if one of the gods would put their arm in his mouth as a sign of good faith. Naturally, none wanted to volunteer as they knew that the game was a rouse and that they would lose their arm.
Finally, the god Tyr agreed. He had been close to Fenrir when he was a pup and thought it was the only fair thing to do.
The man that appears before Fenrir in this image is Tyr, as indicated by the presence of the Tiwaz rune that was sacred to him. This is the moment before he puts his hand in the mouth of the beast.
Fenrir was shackled, and Tyr lost his arm. Fenrir is destined to remain imprisoned until Ragnarok, when he will be able to break his chains and seek his revenge. He will rampage through the nine worlds of the Norse cosmos devouring everything in his path, and eventually devouring Odin himself.
In the meantime, this T-shirt celebrates the nobility and bravery of Tyr, as well as the raw natural power embodied by Fenrir.