Why call it The Hammer of Thor?

In Norse mythology, Mjǫllnir is the hammer of Thor, the god associated with thunder, lightning, storms, oak trees, strength, destruction, fertility, healing, and the protection of mankind in Norse mythology. Distinctively shaped, Mjöllnir is depicted in Norse mythology as one of the most fearsome weapons, capable of leveling mountains. Though generally recognized and depicted as a hammer, Mjöllnir is sometimes referred to as an axe or club.
Ultimately stemming from Proto-Indo-European religion, Thor is a prominently mentioned god throughout the recorded history of the Germanic peoples, from the Roman occupation of regions of Germania, to the tribal expansions of the Migration Period, to his extreme popularity during the Viking Age, where, in the face of the process of the Christianization of Scandinavia, emblems of his hammer, Mjöllnir, were worn in defiance and Norse pagans personal names containing the name of the god bear witness to his flourishing popularity.
After the Christianization of Scandinavia and into the modern period, Thor continued to be acknowledged in rural folklore throughout Germanic regions. Thor is frequently referenced in place names, the day of the week Thursday ("Thor's day") bears his name, and names stemming from the pagan period containing his own continue to be used today.
It is on “Thor’s Day” every week that a swift shot of history dealing with that date will be delivered.  Brace yourself for The Hammer of Thor.