What is Thor's Hammer?
Mjöllnir is the hammer of Thor, a major god associated with thunder 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.
Mjöllnir simply means "crusher," referring to its pulverizing effect. It is related to words such as the Icelandic verbs mölva ("to crush") and mala ("to grind"), and Swedish noun mjöl ("flour"), all related to English meal, mill and miller. Similar words, all stemming from the Proto-Indo-European root *melə, can be found in almost all European languages, e.g. the Slavic melvo ("grain to be ground") and molotu ("hammer"), the Russian Молот (molot - "hammer"), the Greek μύλος (mylos - "mill") and the Latin malleus "hammer", from which English mallet derives, as well as the Latin mola ("mill").
An alternative theory suggests that Mjöllnir might be related to the Russian word молния (molniya) and the Welsh word mellt (both words being translated as "lightning"). This second theory parallels with the idea that Thor, being a god of thunder, might therefore have used lightning as his weapon.
I have one. Mjollnir is tattooed for a number of reasons. First and foremost, as a declaration of one's faith, Heathenry, Asatru or whatever the person prefers to go by, it all boils down to the same thing. Also it is used for protection, strength and vitality (and virility). Or, the person might simply have it because he likes it and doesn't have a clue what it stands for.