The human skeleton is the last memory our body leaves once deceased, unlike the rest of the soft tissues it takes long time to decompose completely. We can list 3 distinct meanings for a skeleton tattoo:
Anarchy: For those who are not willing to compromise with the power and authority of others, a skeleton tattoo conveys a sense of individualism, freedom and independence. Just think of the pirate symbols or riders, who often use it to represent a group.
Respect for the dead: the dead are honored around the world, but the culture that certainly makes the most use of the skeleton image is definitely the celebrated Mexican sugar skulls, another very common artwork in the world of tattoos.
The good luck meaning is used to contrast with the more classical image linked to death. The skeleton can also be used to express enthusiasm and derision toward death itself.