The tattoo meaning of the number 13 can vary person to person. The reason why they have 13 tattooed on them could be for many reasons, it may just be their favorite number, or it could symbolize the 13th letter of the alphabet. (Which is M, and could stand for marajuna) It could also symbolize death or luck, depending on which way you look at it.
The symbolic meaning of number thirteen receives a shady interpretation in Norse myth, when an honorary banquet was held in Valhalla for Baldur (god of nobility, redemption and admirable strength) among other twelve Norse gods. Loki (the trickster) came to the banquet as the uninvited thirteenth guest. Afterwards, we learn Baldur was slain by Höðr who, to execute the god, was given a magic spear by Loki.
Similarly, the darker symbolism of thirteen continues in its Christian association with the Last Supper. Twelve disciples and the Christ, formed a group of thirteen to share the last meal. Here, thirteen is seen as a foreboding omen as we learn of Christ’s fate after the gathering.
In the same vein, Friday the thirteenth is commonly reviled as it was said the Last Supper in the upper room took place on a Friday. Further, Friday 13th in 1307 was the date on which the assembly of the Knights Templar were systematically assasinated.
In the guest company scenarios we can point to Judas or Loki as the thirteenth figures and so deem the association of the number thirteen as unlucky.