Why does Nick Sobotka have a pi tattoo?

Nick Sobotka is a major character in the second season of The Wire. In many scenes we can see that he has a very large letter pi (π) tattooed on the back of his neck. Why does he have this rather unusual tattoo?

I am interested specifically in a story-internal explanation, particularly if it comes from any of the writers, producers, make-up artists, actors, or others involved with the show itself.

Pi is a letter of the Greek alphabet, and is also commonly associated with mathematics (where it is famously used to represent a geometrical constant) and certain "Greek life" fraternities at American universities. But in the show, Nick Sobotka is a relatively poor, presumably uneducated stevedore of Polish descent. It seems unlikely that he has any particular affinity for mathematics, Greek culture, or college fraternities. Does the symbol have any other meaning that would be significant for a character of Sobotka's background?

Possibly it is the character's actor, Pablo Schreiber, who has the tattoo, and the show runners simply didn't bother covering it up. If this story-external explanation is the only one, then it would be great if someone could post an answer confirming it (for example, by showing an off-set photo of Schreiber with the tattoo).