NetHack 3.0.0 was the first version to implement unique artifacts (so no more multiple Snickersnees), and featured several new artifacts, among them the lucern hammers named Thunderfist and Ogresmasher. It seems that the DevTeam forgot that a "lucern hammer" is a polearm, not to be swung as a mallet, but to be thrust at a distant enemy; it was perhaps an error to choose "lucern hammer" as the base type for both weapons. In NetHack 3.0.3, the DevTeam added a new weapon type, the war hammer, switched Thunderfist and Ogresmasher to the new type, and renamed Thunderfist to Mjollnir.
We are not sure of the origin of the name "Thunderfist", 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, the Two-Handed Flail 'Thunderfist'.