In chapter 4, Stanley meets Mr. Sir. Mr. Sir has a rather unique look. Despite being indoors when Stanley meets him, Mr. Sir wears a cowboy hat and sunglasses, and on one of his arms he has "a tattoo of a rattlesnake." Stanley notices, when Mr. Sir is asked to sign some papers, that the rattlesnake appears to "wiggle" when Mr. Sir writes.
A snake is typically associated with evil. In the Garden of Eden biblical story, for example, the snake tricks Adam and Eve into eating the forbidden fruit from the tree of knowledge. The snake is thus an appropriate image to associate with Mr. Sir, as he is a deeply unpleasant, deceptive, and nasty character.
The rattlesnake is also a fitting tattoo for the character of Mr. Sir because there are lots of rattlesnakes in Camp Green Lake. The rattlesnake tattoo on his arm indicates that he has become accustomed to and a part of everyday life in Camp Green Lake.
Later in the story, in chapter 20, Mr. Sir is struck across the face by the Warden. She scratches his face with her nails, which, ironically, are painted with a polish that contains rattlesnake poison. When the poison takes effect, Mr. Sir screams and rolls on the floor in agony. This moment indicates that although Mr. Sir likes to put on a show of being tough (hence the rattlesnake tattoo), he is really not. The Warden is the real rattlesnake character in Camp Green Lake.