What does the number 13 tattoo on your neck mean?

It's related to a Johnny Cash song, so Johnny Cash fans sometimes do this. http://www.lyricsdepot.com/johnny-cash/thirteen.html

* Note: While Johnny Cash did indeed sing the song on his 1994 album American Recordings, it was actually written by Glen Danzig, who allegedly wrote it specifically for Mr. Cash.

Danzig's own version is on the 1999 album "6:66 Satan's Child" and can be heard in the opening credits of the 2009 movie "The Hangover".