NetHack 3.0.0 was the first version to implement unique artifacts and featured several new artifacts - among them the lucern hammers named Thunderfist and Ogresmasher. The DevTeam may have erroneously chosen the lucern hammer as their base items, unaware that it was a polearm. In NetHack 3.0.3, the DevTeam added a new weapon type, the war hammer, switched the base items of both artifacts to it, and renamed Thunderfist to Mjollnir.
The origin of the name "Thunderfist" is uncertain, but it appears in multiple computer games of the fantasy genre; the name tends to have association with both dwarves and fighting. Angband has an artifact called the Two-Handed Flail 'Thunderfist'.