What is the meaning of a thors hammer tattoo?

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.