Two of Mal'akh's tattoos are on his legs. As described in Chapter 2 of The Lost Symbol (page 11 of the English edition):
... his muscular legs were tattooed as carved pillars -- his left leg spiraled and his right leg vertically striated, Boaz and Jachin.
So, what is this about?
For almost three hundred years, it has been public knowledge that two important symbols in Freemasonry are the pillars Boaz and Jachin. Anciently, pillars with these names were part of the Temple built by King Solomon. In the Bible, the construction of the Temple is described in a couple of places, one of which is the First Book of Kings. The 7th chapter, verses 15 through 22, describe the pillars, at the conclusion of which we read:
And he [Hiram, the master builder] set up the pillars in the porch of the temple: and he set up the right pillar, and called the name thereof Jachin: and he set up the left pillar, and called the name thereof Boaz. (1 Kings 7:21)
The name Jachin translates to "God will establish"; Boaz translates to "in strength." Thus, one meaning of the pillars is that they signify that God established the Temple in strength.
There are ancient legends associated with the pillars, some of which have shown up in Masonic symbolism. One legend is that the pillars were topped with globes, one showing the globe of the Earth ("the terrestrial globe") and the other the globe of the heavens as seen from the Earth ("the celestial globe"). You can see these globes atop the pillars in the illustration above (from a 1920's edition of Albert Mackey's History of Freemasonry). Another legend is that there was secret wisdom written on records deposited within the pillars. Yet another legend (shown in Dan Brown's novel) is that one pillar was decorated with carvings horizontally, the other vertically.
What is the significance of the pillars in Masonic symbolism, and why are they of interest to Mal'akh? There is much that could be said about this, only a portion of which I can mention here.
The pillars Boaz and Jachin signify the entrance to the Temple built by Solomon and dedicated to God. Some representation of these pillars is found in most Masonic lodge rooms. Their presence helps to mark the Masonic lodge room as a sort of sacred space, where people take a different perspective on their lives than the everyday point of view. The "journey" of initiation into Freemasonry begins as the initiate symbolically enters the Temple built by Solomon.
The names of the pillars are a reminder to the Mason that, as the outward Temple was established in strength by God, so too the Mason invites God to establish his own life in strength. (Of course, if the Mason invites God to establish his life, he'd better be living the kind of life that God can support.)
As the globes atop the ancient pillars were representations of the heavens and the earth, so too the Mason seeks for knowledge of the natural universe, near and far.
As the ancient pillars were said to be repositories of wisdom, so too the Mason is on a journey in search of wisdom.
These are the values that Mal'akh was dedicating himself to: the search for wisdom. The tragedy of Mal'akh, in part, is that he did not understand that the search for higher wisdom requires kindness and respect for other human beings, in order to be a fully successful search.
I shall have a great deal more to say about Masonic symbolism as it appears in Dan Brown's novel in my forthcoming book, Discovering The Lost Symbol: The Mind of Dan Brown, the Truth About the Freemasons, and the Idea That We Can Become Gods. (Agents' and publishers' inquiries are welcome.)