User blog comment:Ryuske/Luffy's tattoo/@comment-1595565-20100802231442

I'm pretty sure it's a tattoo, there's no bandages on that part of his arm, and that part could not be seen when he was lying down. As for what it says, I see: a crossed out S, an O D or P, then the third one looks like a B or R, and the fourth looks like an A crossed out. So, it could be Sora or Soba, which mean sky and wheat respectively.